I have both. The Xbox 360 shits all over the PS3, but only at the moment. I believe the PS3 has more potential, but if you need a console now, go for a 360.
Resistance: Fall of Man - looks crap compared to most Xbox 360 FPS
Motostorm - graphically, looks about the same as DiRT (which is both on 360 and ps3 soon [it often takes a while for games to be ported to the ps3)
Heavenly Sword - looks good, but not fantastic. Fun to play, but short.
Ninja Gaiden - orsum
Sports games, such as madden: worse on the PS3 at the moment. Firstly because of a lack of vibration in the controller. Secondly, they're getting more graphical glitches and lower frame rates (although I'm sure in a year's time these will be sorted out)
I think the main problem is that PS3 games are taking longer to develop. For instance, I will be playing Stranglehold next week. PS3 owners must wait until the end of the month. I'll be playing Half-Life: Orange Box 3 weeks before the PS3 version. DiRT about 3 months before the PS3 version. The list of such titles is absolutely huge.