AC2 was awesome but I finished it pretty damn quickly... but then the first Bioshock wasn't exactly long either. Meh, get both. It doesn't matter which you buy first, they're both good.
ip dip doo the cat stole the shoe the doggie ate a biscuit and the parrot got the flu so i choose you bioshock 2
It's gotten fantastic reviews, but it's highly recommended that you play the first, though. Not necessary, but if you want, you can get Bioshock and Mass Effect for $30.
It's quite good, in fact: ME2 is easily the best game I've played this generation, outside of maybe Mario Galaxy. Do you like phenomenal gameplay, audio, story and replay-value? This game is for you.
Yes, it is the correct answer! But what if the OP doesn't have a PS3? Or doesn't have Uncharted 2?!!! Also, there's DLC out now.