I actually prefer the PS3, but I definitely understand the 360 love. The controller for the 360 is sturdier and more ergonomically designed. The Dual Shock 3 feels kind of like a cheap toy next to it. The XBL environment is more robust than the PSN and in the US, most gamers are on 360. So when you want to game with your friends, you do it on 360.
I, however, prefer the exclusives on PS3 like Uncharted, Journey and The Last of Us.