I've seen a lot of threads popping up lately about picking a new army, so I thought I'd make a sticky topic with some general guidelines for deciding where to throw your lot in the 41st Millenium.
There are a few questions you need to ask yourself before you get started.
-How much am I willing to spend?
-How big an army do I want / how big an army do I need to play at the size my group plays at?
-Which models do I like?
-Am I more concerned about winning the game at all costs, or am I out for a good time?
-Am I a painter/modeller before a gamer, or a gamer before a painter/modeller?
-How long have I been involved in the GW hobby?
Financially, any army CAN be easy on your wallet, as these things go. Necrons are by far the most inexpensive army to build financially, as the Battle Force boxes contain pretty much everything you'll need. It's just a question of how many Battle Forces you'll need to build your army to the appropriate size, and then getting a couple Necron Lords. Space Marines and Chaos Space Marines of all flavors are probably the next in line on the financial ladder, followed then by pretty much everyone else.
I've found that in short order, you'll lose interest in an army if you think their models are lame, even if they play really well. There are no bad armies in the game in a gaming sense, so you might as well pick something you'll like modelling, painting, and having to look at.
Your choice of models, when you've selected your army, is really a question of whether you're building an army list specifically to win, or if you'd just as soon build some units to a vague points cost, figure out the concrete details later, and have fun with it. If your approach is a more competitive one, then you'll probably be happier building the army to a pre-set army list that's been designed to take on all comers equally well, using the trump cards that the given army has to offer. A more casual approach often leaves you with more models that you won't use every game, but also tends to make a more fluffy army in the end.
Some models are simply harder to paint than others. If you fancy a challenge painting, Eldar might be up your alley. If you're not much for modelling and painting, then Necrons might be for you. Some people are bothered by not having painted armies, some aren't. It's a question of how much work you want to put into the army, and whether or not you consider it a chore. Armies that are painted because you like to paint come out looking far sharper on average, but you can still make a very good looking Space Marine, Tau, or Necron army without putting a whole lot of effort into the models if you aren't too big on the hobby side of the hobby.
Picking the right army for your experience helps out a fair bit as well. Here's a rough breakdown of how I look at armies overall, in terms of learning curve, ease of modelling, and whatnot.
Chaos Space Marines-- Very easy to learn, though difficult to master. A great starting point in the hobby, as you're free to paint them and model them however you like-- wacky things don't look too out of place most of the time, so if you screw up on a model, no one is likely to notice. Financially, Chaos is on the cheaper end of things as well. On the table, a Chaos army's advantages are good armor saves, exceptional close combat ability, and excellent basic troops; while their disadvantages tend to be a low model count. Chaos Space Marines are individually extremely tactically flexible, as they're armed with a boltgun, bolt pistol, and close combat weapon. In 5th Edition, this lets them easily adapt to either a more ranged firefight or fight in close combat as needed-- the range that 5th Edition takes place at makes Chaos Space Marines quite possibly the most flexible individual models in the game at the moment.
Daemonhunters-- On the more difficult end of things to play with, as the model count is phenominally low. They're a tad bit pricey as most of the models are metal, as well. Daemonhunters offer a huge array of modelling options, though, as no two Inquisitors and their retinues are alike. On the table, a Daemonhunter army's advantages are a wide array of specialized abilities, high ability values, and impressive anti-infantry firepower, while disadvantages lie in overspecialization and low model count. In the new edition of 40k, Grey Knights come into a new element, as they're optimized for the intense, close-range pace that the game has taken on. Powerful template weapons, such as the incinerator, are also at the daemonhunter's disposal, which makes them a potent choice for 5th Edition games.
Daemons of Chaos-- A few things stand out-- Daemons can be played with an identical army list in both 40k and Fantasy, as far as model selection goes, so you can get a two for one deal with them, in essence, as long as you mount them on square bases. Barring it being an official Games Workshop event, nobody will say anything if you have daemons on square bases in the 41st Millenium. In 40k, Daemons are a highly specialized army, and take a bit of finesse. There's not a whole lot of shooting, and you have to rely a little bit on luck to determine which half of your army decides to show up first. Based on what I've seen so far, I'd reccomend them to someone who's been in the hobby a while. Perhaps not as long as someone who should be playing Eldar or Sisters of Battle well, but still not a new gamer. The Daemons of Chaos army's advantages on the table are exceptional close combat ability with the ability to quickly engage before the opponent can really react; with disadvantages primarily being low shooting and only a 5+ invulnerable save to protect most models.
Dark Eldar-- The general consensus is that Dark Eldar are the hardest army to play with, and have an extremely steep learning curve. They're Direct-Only for the moment, so getting your hands on them requires a little extra effort. The models are old, and show their age-- they weren't great, even for their time. If that doesn't bother you, though, the army is possibly the most devastating in the game in the hands of a skilled player. The Dark Eldar army's advantages are speed and short-range firepower, disadvantaged by fragility and low model count.
Eldar-- Eldar are a painter's army. Each squad type has its own distinctive costume, and you can really go to town on Harlequins and some of the grav vehicles. A lot of the range is metal, so they're kind of expensive. The army is pretty fragile, too-- not quite as much as Dark Eldar, but close. It's difficult to win with Eldar if you don't know what you're doing. They're undeniably the best army for someone who loves to paint, though. The Eldar army's advantages are speed, close range firepower, and close combat ability. That comes at the price of a very fragile army and exceptionally high points values per model.
Imperial Guard-- The Imperial Guard can go a lot of different directions. With a new Codex, the Guard player is offered a wider array of options than they were before, even with the removal of the Doctrines rules from this edition. Heavy armour can be fielded in squadrons, and the Imperial Guard player has access to over 20 different kinds of tanks. If you're a treadhead, this may be the army for you. The Imperial Guard also is an incredible army as mass infantry, although a pure infantry Imperial Guard army is going to cost more than most pewter armies due to the volume of men you need. The Guard offer a lot to players who like historical armies-- it's not hard to model up an Imperial Guard army to look like a World War I or II army, by any means. The Imperial Guard's advantages are high model count and a huge volume of heavy weapons. It's possible for a Guard army to fire 300 lasgun shots alone in the shooting phase, plus dozens of heavy weapons on top of that! Their disadvantages are very strictly average (some would even say poor, but they're acceptable for their points) individual troopers and a relative lack of mobility.
Necrons-- Necrons are sort of the antithesis of Eldar. They're outrageously simple to build and paint, have very little variety, and have a tremendous amount of staying power. They're extremely forgiving on the tabletop, and extremely inexpensive to build financially. If you don't mind the slightly dry nature of the army, they're probably the best starting point in the game these days. The main advantage of the Necron army is its durability, offset by low model count.
Orks-- Orks are pricey to build, just because you need so many Boyz, but the army itself is rather forgiving tactically. In the end, an Ork is an Ork, and all that matters is that he gets himself stuck into close combat, which is simple enough. Painting can get a little monotonous with Orks, though-- green isn't difficult, but there are a hell of a lot of them to do. Orks excel in close combat, and have high numbers in their armies, although they're poor shots and are very vulnerable to being shot with some of the worst armor saves in the game. In 5th Edition, Orks both dish out and take it in equal measure. Large mobs are extremely destructive, but an individual Ork isn't that hard to kill in the grand scheme of things, so losing combat with fearless mobs becomes a very costly endeavor.
Space Marines-- Generally speaking, most people point new players towards the Emperor's Astartes. They're a solid army, and are the poster-boys of Games Workshop. They're easy to learn, but it takes years to truly master playing with them. They occasionally seem a bit dry, and other Space Marine armies are a dime a dozen, but you'll never have to worry about having an inferior or out of date army. You've also got the most options rules-wise with Space Marines-- there are five different Codecies available for them. Space Marines are almost entirely plastic, which makes learning to model pretty simple. Marines are relatively inexpensive financially, as well-- not so much as Necrons, but about on par with Chaos Space Marines. The advantages of a Space Marine army are excellent basic troops coupled with good shooting and close combat ability across the board, as well as being impossible to rout. They suffer in protracted combats due to low model count, however. The new Codex: Space Marines eliminates Chapter Traits; but in exchange offers an unprecidented array of equipment for combating very specific threats at any range.
Tau-- If you're an anime addict, particularly of the mecha persuasion, then Tau are probably right up your alley. They're easy to paint, fairly forgiving, and carry a moderate price tag. They've got the most powerful ranged weapons in the game. Their only real weakness is in close combat, but they have measures which make it quite difficult to engage them. It's unclear these days whether Tau or Orks are the army to beat. Tau are probably third down on the list as far as good starter armies, behind Necrons and Space Marines/Chaos Space Marines. Tau armies are highly mobile and boast the strongest weapons on basic infantry in the game, but are generally extremely poor in close combat.
Tyranids-- Tyranids offer a lot of possibilities on the modelling front. They're rather difficult to paint well, however, and are among the most expensive armies to buy simply because of how Gaunts are packaged. They've got a moderate learning curve, and getting the right balance of critters in the army takes some time. Tyranids are, ultimately, a very powerful army, though, and rewarding if you take the time to model them and learn to play them well. A Tyranid army's advantages tend to be high model count, exceptional close combat ability, and complete fearlessness. Their disadvantages are vulnerability to shooting due to poor armor saves.
Witch Hunters-- Sisters of Battle are downright pricey, mostly because they're all pewter. The Adepta Sororitas are a close-range powerhouse, though, and are good if you want the 3+ armor save without being a Space Marine. Sisters have a steep learning curve, mostly because it takes some time to learn when to execute Acts of Faith. Sisters, like Eldar, seem to be a more veteran gamer's army. The Sisters of Battle army's advantages are Acts of Faith and powerful short-range shooting combined with excellent armor saves. They are disadvantaged by strictly average skills and very limited long range shooting ability.
There are a few questions you need to ask yourself before you get started.
-How much am I willing to spend?
-How big an army do I want / how big an army do I need to play at the size my group plays at?
-Which models do I like?
-Am I more concerned about winning the game at all costs, or am I out for a good time?
-Am I a painter/modeller before a gamer, or a gamer before a painter/modeller?
-How long have I been involved in the GW hobby?
Financially, any army CAN be easy on your wallet, as these things go. Necrons are by far the most inexpensive army to build financially, as the Battle Force boxes contain pretty much everything you'll need. It's just a question of how many Battle Forces you'll need to build your army to the appropriate size, and then getting a couple Necron Lords. Space Marines and Chaos Space Marines of all flavors are probably the next in line on the financial ladder, followed then by pretty much everyone else.
I've found that in short order, you'll lose interest in an army if you think their models are lame, even if they play really well. There are no bad armies in the game in a gaming sense, so you might as well pick something you'll like modelling, painting, and having to look at.
Your choice of models, when you've selected your army, is really a question of whether you're building an army list specifically to win, or if you'd just as soon build some units to a vague points cost, figure out the concrete details later, and have fun with it. If your approach is a more competitive one, then you'll probably be happier building the army to a pre-set army list that's been designed to take on all comers equally well, using the trump cards that the given army has to offer. A more casual approach often leaves you with more models that you won't use every game, but also tends to make a more fluffy army in the end.
Some models are simply harder to paint than others. If you fancy a challenge painting, Eldar might be up your alley. If you're not much for modelling and painting, then Necrons might be for you. Some people are bothered by not having painted armies, some aren't. It's a question of how much work you want to put into the army, and whether or not you consider it a chore. Armies that are painted because you like to paint come out looking far sharper on average, but you can still make a very good looking Space Marine, Tau, or Necron army without putting a whole lot of effort into the models if you aren't too big on the hobby side of the hobby.
Picking the right army for your experience helps out a fair bit as well. Here's a rough breakdown of how I look at armies overall, in terms of learning curve, ease of modelling, and whatnot.
Chaos Space Marines-- Very easy to learn, though difficult to master. A great starting point in the hobby, as you're free to paint them and model them however you like-- wacky things don't look too out of place most of the time, so if you screw up on a model, no one is likely to notice. Financially, Chaos is on the cheaper end of things as well. On the table, a Chaos army's advantages are good armor saves, exceptional close combat ability, and excellent basic troops; while their disadvantages tend to be a low model count. Chaos Space Marines are individually extremely tactically flexible, as they're armed with a boltgun, bolt pistol, and close combat weapon. In 5th Edition, this lets them easily adapt to either a more ranged firefight or fight in close combat as needed-- the range that 5th Edition takes place at makes Chaos Space Marines quite possibly the most flexible individual models in the game at the moment.
Daemonhunters-- On the more difficult end of things to play with, as the model count is phenominally low. They're a tad bit pricey as most of the models are metal, as well. Daemonhunters offer a huge array of modelling options, though, as no two Inquisitors and their retinues are alike. On the table, a Daemonhunter army's advantages are a wide array of specialized abilities, high ability values, and impressive anti-infantry firepower, while disadvantages lie in overspecialization and low model count. In the new edition of 40k, Grey Knights come into a new element, as they're optimized for the intense, close-range pace that the game has taken on. Powerful template weapons, such as the incinerator, are also at the daemonhunter's disposal, which makes them a potent choice for 5th Edition games.
Daemons of Chaos-- A few things stand out-- Daemons can be played with an identical army list in both 40k and Fantasy, as far as model selection goes, so you can get a two for one deal with them, in essence, as long as you mount them on square bases. Barring it being an official Games Workshop event, nobody will say anything if you have daemons on square bases in the 41st Millenium. In 40k, Daemons are a highly specialized army, and take a bit of finesse. There's not a whole lot of shooting, and you have to rely a little bit on luck to determine which half of your army decides to show up first. Based on what I've seen so far, I'd reccomend them to someone who's been in the hobby a while. Perhaps not as long as someone who should be playing Eldar or Sisters of Battle well, but still not a new gamer. The Daemons of Chaos army's advantages on the table are exceptional close combat ability with the ability to quickly engage before the opponent can really react; with disadvantages primarily being low shooting and only a 5+ invulnerable save to protect most models.
Dark Eldar-- The general consensus is that Dark Eldar are the hardest army to play with, and have an extremely steep learning curve. They're Direct-Only for the moment, so getting your hands on them requires a little extra effort. The models are old, and show their age-- they weren't great, even for their time. If that doesn't bother you, though, the army is possibly the most devastating in the game in the hands of a skilled player. The Dark Eldar army's advantages are speed and short-range firepower, disadvantaged by fragility and low model count.
Eldar-- Eldar are a painter's army. Each squad type has its own distinctive costume, and you can really go to town on Harlequins and some of the grav vehicles. A lot of the range is metal, so they're kind of expensive. The army is pretty fragile, too-- not quite as much as Dark Eldar, but close. It's difficult to win with Eldar if you don't know what you're doing. They're undeniably the best army for someone who loves to paint, though. The Eldar army's advantages are speed, close range firepower, and close combat ability. That comes at the price of a very fragile army and exceptionally high points values per model.
Imperial Guard-- The Imperial Guard can go a lot of different directions. With a new Codex, the Guard player is offered a wider array of options than they were before, even with the removal of the Doctrines rules from this edition. Heavy armour can be fielded in squadrons, and the Imperial Guard player has access to over 20 different kinds of tanks. If you're a treadhead, this may be the army for you. The Imperial Guard also is an incredible army as mass infantry, although a pure infantry Imperial Guard army is going to cost more than most pewter armies due to the volume of men you need. The Guard offer a lot to players who like historical armies-- it's not hard to model up an Imperial Guard army to look like a World War I or II army, by any means. The Imperial Guard's advantages are high model count and a huge volume of heavy weapons. It's possible for a Guard army to fire 300 lasgun shots alone in the shooting phase, plus dozens of heavy weapons on top of that! Their disadvantages are very strictly average (some would even say poor, but they're acceptable for their points) individual troopers and a relative lack of mobility.
Necrons-- Necrons are sort of the antithesis of Eldar. They're outrageously simple to build and paint, have very little variety, and have a tremendous amount of staying power. They're extremely forgiving on the tabletop, and extremely inexpensive to build financially. If you don't mind the slightly dry nature of the army, they're probably the best starting point in the game these days. The main advantage of the Necron army is its durability, offset by low model count.
Orks-- Orks are pricey to build, just because you need so many Boyz, but the army itself is rather forgiving tactically. In the end, an Ork is an Ork, and all that matters is that he gets himself stuck into close combat, which is simple enough. Painting can get a little monotonous with Orks, though-- green isn't difficult, but there are a hell of a lot of them to do. Orks excel in close combat, and have high numbers in their armies, although they're poor shots and are very vulnerable to being shot with some of the worst armor saves in the game. In 5th Edition, Orks both dish out and take it in equal measure. Large mobs are extremely destructive, but an individual Ork isn't that hard to kill in the grand scheme of things, so losing combat with fearless mobs becomes a very costly endeavor.
Space Marines-- Generally speaking, most people point new players towards the Emperor's Astartes. They're a solid army, and are the poster-boys of Games Workshop. They're easy to learn, but it takes years to truly master playing with them. They occasionally seem a bit dry, and other Space Marine armies are a dime a dozen, but you'll never have to worry about having an inferior or out of date army. You've also got the most options rules-wise with Space Marines-- there are five different Codecies available for them. Space Marines are almost entirely plastic, which makes learning to model pretty simple. Marines are relatively inexpensive financially, as well-- not so much as Necrons, but about on par with Chaos Space Marines. The advantages of a Space Marine army are excellent basic troops coupled with good shooting and close combat ability across the board, as well as being impossible to rout. They suffer in protracted combats due to low model count, however. The new Codex: Space Marines eliminates Chapter Traits; but in exchange offers an unprecidented array of equipment for combating very specific threats at any range.
Tau-- If you're an anime addict, particularly of the mecha persuasion, then Tau are probably right up your alley. They're easy to paint, fairly forgiving, and carry a moderate price tag. They've got the most powerful ranged weapons in the game. Their only real weakness is in close combat, but they have measures which make it quite difficult to engage them. It's unclear these days whether Tau or Orks are the army to beat. Tau are probably third down on the list as far as good starter armies, behind Necrons and Space Marines/Chaos Space Marines. Tau armies are highly mobile and boast the strongest weapons on basic infantry in the game, but are generally extremely poor in close combat.
Tyranids-- Tyranids offer a lot of possibilities on the modelling front. They're rather difficult to paint well, however, and are among the most expensive armies to buy simply because of how Gaunts are packaged. They've got a moderate learning curve, and getting the right balance of critters in the army takes some time. Tyranids are, ultimately, a very powerful army, though, and rewarding if you take the time to model them and learn to play them well. A Tyranid army's advantages tend to be high model count, exceptional close combat ability, and complete fearlessness. Their disadvantages are vulnerability to shooting due to poor armor saves.
Witch Hunters-- Sisters of Battle are downright pricey, mostly because they're all pewter. The Adepta Sororitas are a close-range powerhouse, though, and are good if you want the 3+ armor save without being a Space Marine. Sisters have a steep learning curve, mostly because it takes some time to learn when to execute Acts of Faith. Sisters, like Eldar, seem to be a more veteran gamer's army. The Sisters of Battle army's advantages are Acts of Faith and powerful short-range shooting combined with excellent armor saves. They are disadvantaged by strictly average skills and very limited long range shooting ability.