PS3: Has Blu-Ray player if you don't have one already.
Xbox360: Has better multi-player experience
Other than that, both consoles are pretty much identical from a performance point of view.
The one major difference is that for multi-platform games, companies tend to lead with the 360 version as the 360 is much easier to code for, that means occasionally the PS3 version is not quite as good (again, it's marginal though)
At the end of the day, it's down to which platform you prefer, and the console exclusives you like the look of most.. Although if you have mates you like to game with it's probably best to get the same as them so you can multiplayer with them.
FWIW, I have loads of mates who have both machines, but the huge majority of them spend most of their time on the 360 purely because of the decent, integrated multiplayer experience.