I have both (thanks to an amazing Christmas last year, thanks hunny!), and you can't go wrong with adding a PS3 to your collection. I mainly use it for sports games (better graphics), movies (so bloody clear!), and those few exclusives I like (InFamous is amazing).
XBox is still great for anything you're going to play on Live, though.
Ironically, with the 360 and the PS3 I'm playing old PC games ... (though it's KOTOR 2).
"God, my brilliance is now somewhat of a burden. Get back to me."