they're both great games and you can't go wrong with either of them IMO. you can get bioshock for a pretty cheap price. i think you can buy a copy new at gamestop for around 20 bucks now. fallout 3 also has a game of the year edition for 60 bucks that includes the orginal game as well as all the downloadable content released for it. it's a great buy considering all the amout of content included in it. the regular edition i think is priced around 30 bucks now
it all just depends on how much you're willing to spend and what you're looking for. if you want to experience one of the best single player experiences with some of the best graphics the 360 has to offer for a cheap price, go with bioshock. if you're looking for one of the greatest RGP's on the 360 with a "choose your own destiny" type storyline, go with fallout 3. if you're willing to pony up with extra cash, though, i don't think you could go wrong with the game of the year edition considering how much extra stuff is included.