I've had a Wii for awhile, and recently decided that it was about time I grabbed an Xbox 360 or a PS3. Did a bunch of research on the differences between the two, and here's the verdict, the way I see it:
PS3 -- Go with this console if you're looking for great single-player campaigns / adventures.
Xbox 360 -- Multiplayer, either local or online, is where this console seems to shine.
Of course, I'm just comparing the game libraries themselves. If you're concerned with peripheral features then YMMV.
I ultimately decided on the Xbox 360, considering that I'll probably be spending a lot of time playing with friends.