Chaos marines may share the same armor save and SOME (mind you the CSM's get the short end on every vehicle) vehicles, but they play so much differently.
first off.
CSM's don't have drop pods, so our units have to either slog it to the enemy, or ride some spikey metal boxes like drunken southerners and bring the whoopin to the enemy.
secondly, the psychic powers available to chaos are VERY different to the loyalist goodie two shoes (some of our powers suck, but theres deliciously evil ones as well)
Thirdly:
Units! The space marines are limited to pre-costed squads and can't go higher then 10, also EACH of their units are designed for a specific job. Chaos marines start off with every small upgrade (including a bolter, cc weapon, AND a pistol!) and can be upgraded any way you want to fit each job (a massive mob of 20 marines just to walk around smacking stuff for instance)
Chosen marines are like terminators without the 2+ save or Deep strike, instead they get some nasty weapon options as well as being able to infiltrate! (5 melta guns that could start 12 inches away from your tanks on turn 1! OUCH! killed 3 land raiders on turn one this way with three chosen squads)
Chaos terminators are cheaper versions of their loyalist cousins, but they lose the options for plasmacannons, assault cannons or stormsheilds/thunderhammers. this makes them rather weak in comparison, but if you make them all champions, give them mark of khorne, and twin lightning claws, you've got terminators that have 6 power attacks each on a charge (they even get re-rolls to wound!) put em in a land raider and get em into combat, they will demolish anything they assault
Possessed marines are a piece of flavor, they can be good assaulters, tank killers, fleeting monsters, or tough crackers depending on the roll but thats about it, risky.
ICONS! Space marines don't get these, and depending on the icon they buff your chaos marines (some good, some expensive, but all deadly, ever seen 20 marines with a 5+ invuln or 4 attacks each on a charge, or marines that have 5 initiative or toughness? crazy)
CULT TROOPS:
Chaos marines get four types of troops that cost more then normal marines, but they each come with POWERFUL abilities.
Plague marines are tough crackers that get FNP but lose an initiative. they are able to take two special weapons at under 10 men!
Khorne berzerkers get an extra attack AS WELL as furious charge (these guys destroy what ever they charge),
Thousand sons are S&P but they gain a 4+ invuln, psychic power thanks to the aspiring sorcerer (even a force weapon!, its like a mini librarian!) AS WELL AS AP3 BOLTERS, watch out MEQ armies!
Noise marines are like normal marines but they lose the normal weapon option, instead they gain an initiative and are the only units in the chaos marines able to take sonic weaponry (these include the sonic blaster which is a bolter that can fire twice on the move up to 24 inches, or three times if they didn't move! (10 noise marines pumping out 30 shots a turn are devastating, but expensive) the blastmaster is a heavy bolter that loses a shot, but gains pinning, it is also able to drop a small blast battle cannon!, the final weapon they can take is the doom siren, which is an extremly powerful flamer (S5 ap3 is TERRIFYING)
vehicles:
As said before, the chaos marines are at a huge disadvantage when it comes to vehicles. Heres a list.
CSM's only get a vanilla land raider (mind you its strong, but I would KILL for a crusader)
CSM's don't get razorbacks (these would be lovely for smaller squads and able to bring some decent guns)
CSM's don't get drop pods even though these have existed long before the heresy. DREAD CLAWS DON'T COUNT
CSM's don't get land speeders (yet another pre-heresy vehicle some how unavailable to us)
CSM's don't get manticores, medusas, or any of the like (don't know why, can't we just steal them?)
CSM's dreadnaughts are 10 points cheaper, but they now have a chance to go apeshit and kill your own men. not much of a fair deal if you ask me.
CSM Predators are terribly expensive and suck to boot
and finally.. CSM's don't get the GODLY power of the machine spirit (which makes our landraiders all the worse) and it is replaced with an option to buy daemonic possession (which is nice, but machine spirit was free..)
now for chaos only vehicles/heavies
Chaos marines get Obliterators
Basically a living gun-shop able to fire up to 6 different weapons, sadly these guys are WTFBBQ expensive (75points for a T4 two wound model with a 2+ 5++ save, yes hes got godly armour, but all it takes is one lucky shot to ruin these guys)
don't get me wrong, obliterators are powerful. but expensive when you lose them.
Chaos marines get defilers
Basically a dreadnaught thats been pimped out and given a massive model (this means hes hard to hide) with a powerful built in battle cannon and two extra arms of destruction (able to swap for cc arms, twinlinks reapers, or lascannons but hes still only av 12 as well as BS 3, making him an easy kill. But defilers come with daemonic possession for free, and fleet.
Defilers are besically an expensive dreadnaught capable of long range superiority or short range massacre (two extra cc arms = 7 str 10 armor denying attacks of death!)
The chaos marines final difference is daemons!
The summoned daemons are cheap and expendable units yes they suck but throw them onto an enemy and they are great fodder. the greater daemon is the same. not very good due to the fact that chaos marines can't have god-specific daemons (I would KILL for a keeper of secrets or a abominable great unclean one) being a monstrous creature capable of squishing a few things with ease has its benefits.
and that's about it.
Now tell me are they still similar?
