Hmm, we own both consoles here (allthough the 360 recently died), playtime is divided about equal but I simply prefer the games on different consoles sometimes. For me, its mostly down to controls, I don't care about achievements at all and the visuals and features are often completely alike.
For racing games and beat 'em up's I prefer the PS3 due to the Dualshock 3 controller and for 3rd person shooters I much prefer the 360's controller.
Simple as that.