Personally I prefer the PS3. The graphics are marginally better, supposedly - but that's not an issue for me. The main reasons I use the PS3 over the 360 are the variety of games (Xbox 360 has been, for me, a long line of FPS games and not much else) and the incredibly user-friendly styling it has. I can't stand the Xbox 360 dashboard, it's really ugly. If a game is on both consoles, I'll buy it for the PS3. Oh, another thing I buy games for the PS3 is online gaming. Because it's free, unlike on the 360.
I have both consoles though as there will always be fantastic games no matter which way you go (I consider both Uncharted and Mass Effect series as unmissable). But if you really, really like FPS games then the Xbox 360 is possibly the better opti on overall.