Xbox 360 hands down. I have both and if a game gets released on both platforms i buy it for 360 for 2 reasons; 1. Xbox live 2. Cross platformers usually run better on 360. Live is big for me because my friends from school play it, but also a bunch of people i've met online and I know if i get a game i will usually have atleast a few friends on to play with (although this was a problem with CoD4 because i had to many people to play with and it was hard to choose lol). And although the cross platform issue may not be true with every game, it sure as hell seems to be pretty bad in sports games like madden and apparently orange box is terrible on ps3, atleast thats what i heard.
Honestly what it should come down to is A) Which your friends have and if youll be able to play online with them, if this doesn't apply to you then throw it out but if it does id rank it as a higher priority B) Do you see yourself buying/watching blu rays. No i wouldn't base my choice to go with a ps3 only on blu-ray but it is nice to have C) Probably the most important is which exclusives look better to you. Are you jonsing for some Uncharted, warhawk, haze (when its released), MGS4(when its released), resistance and rachet and clank. Or would you rather be playing Mass Effect, Bioshock, Halo 3, Gears of War, Ninja Gaiden 2(WR) and any other games you may be into. Also I don't know if you have a good PC or not but some of the games like Gears or as you said bioshock are on that as well and if you can play them there then there exclusivity might not hold as much weight. D) Finally the last thing that comes to my mind would be if you have a PS2 and would like to play those games on it. This isn't a huge deal since you could keep your ps2 anyway (i sold mine on craigslist for 50 bucks though).
So there you go, sorry for the huge wall of text but hope it helps some.
Edit: Also incase you didn't know UT3 is coming to xbox next year too, but if your computer can handle bioshock i'd assume it could do UT3 and i'd go with it for PC over a console and thats rare for me.