I'd get the xbox 360.
Mass Effect, Gears of War, Halo 3, Fable 2, Guitar Hero 2 (If you don't already own it for the PS2), Grand Theft Auto IV, Alan Wake, Banjo-kazooie, Blue Dragon, Crackdown, Halo Wars, Too Human, Viva Pinata, Ace Combat 6
The library is just better imo. And considering that the 360 has the price advantage and the one year head start, the library will probably stay better. The only reason to get a Playstation 3 would be if you were in love with some of their first party stuff.
Good luck with the decision.
@Fatal Cobra: Why exactly will the PS3 be better in the longrun?