A tarpit is a unit that does not kill a lot of the opposing unit but survives a longer period of time (either because the models are cheap and thus you can lose a lot of models and still hold on or because the models are difficult to kill) and holds that unit up long enough for another unit to get into its flank or to prevent the unit from doing a lot of damage.
is an excellent Internet site dedicated to Lizardmen) are not a classic tarpit army, except for a Temple Guard unit with a Slann in the second rank or a stegadon (which is stubborn). They have good core and special infantry in saurus and Temple Guard (elite saurus warriors), respectively, and excellent cheap shooting/skirmish units with poison shooting skinks (poison autowounds on rolling a 6 to hit) and salamanders that can march and send out flame template shots. Lizardmen have one of the best, if not the best, lords in the game in the Slann with some of the best access to magic and abilities. Lizardmen also have good fighting lords/heroes in oldbloods and scar vets and a decent hero level caster in a skink priest. The key features of Lizardmen is the scaly skin armour save boost for the saurus, temple guard (elite suarus) and stegadon (monsters that can be ridden); the cold-blooded rule allowing one to roll 3 dice and throw away the least favorable one on all leadership/break tests; the availability of poison shooting units with speed and skirmish abiity; low initiative (means they hit later than opponents) for infantry. Thus, the army has answers to most other armies it may face.
Empire (warhammer-empire.com is the dedicated Internet site for the army with lots of history but kind of hit or miss currently) has classic tarpits in a large block of relatively cheap infantry that can remain steadfast, stubborn elite infantry in greatswords, stubborn elite cavalry in Reiksguard and special stubborn (war altar) or unbreakable units (steam tank and flagellents). Empire, in the new book, can be run with a heavy cavalry focus or a heavy infantry focus or a mix of the two. Core cavalry units can have the best armour save possible (1+) or great weapons with a 2+ AS and one core cav unit can be upgraded to obtain strength 4. Empire has good access to war machine shooting, with cannons and hellblaster volley guns being the best choices. Unique aspects of empire include war priests [arch lectors at the lord level] that give the unit they are in hatred (re-roll to hit on the first round of combat) and can cast cheap "blessings" on the units they are in [to get a ward save of 5+ or re-roll to wound abiilty or flaming attacks], access to good war machines and engineers; generals and bsb captains are only okay but can be heavily armoured and allow their units to roll 3 dice (throwing out worst one) to pass break tests; the detachment special rule that allows certain infantry units (parents) to pass certain abilities to their detachments and the detachments have certain countercharge and stand and shoot abilities in return when the parent unit is charged; unique chariots the provide augments to units in clsoe proximity in the war altar (hatred and war priest blessings), hurricanium (+1 to hit), and luminark (6+ ward save); and heavy plate armour on special infantry, certain characters, and cavalry units allowing for superior armour saves. Empire battle wizards have access to all the common magic lores but do not have an army book lore. The nubmer of options can be bewildering to a new player and can increase the financial commitment to building this army if you want a number of the competition options.
Financial and competitive considerations:
Lizardmen are the most competitive army in the game (top tier) on average because of the Slann, salamanders and flexibility of the army. However, you can expect a new army book within two years and that significant changes in a new book will likely result in having to buy more models and render some of you current army obsolete.
Empire have the newest army book and lots of options. Thus committing to Empire will result in being able to play the army for a number of years before worrying about significant changes in the rules requiring changing the army you have built. Empire is a competitive army with certain builds but not over-the-top or top tier like Lizardmen current are able to be constructed. Empire's large number of options and special units can mean a larger financial commitment and more assembly and painting unless you are able to identify and focus on a specific build and find it successful right away.