I have tried both the xbox 360 and the PS3 and personally myself I prefer the Xbox 360, I have heard that since I tried it though the PS3 has changed a little.
I think with there being more of a range of games and also with there being more people going for teh 360 than the PS3 it woudl mean more gamers online on xbox than PS3.
I remember when I tried the PS3 the game I tried was Rainbow Six Vegas, I had previously tried this on the Xbox 360 and found the graphics to be better on the 360 than on the PS3