If you want different games and want to have new experience (as opposed to the 360), you're better off with the Wii. If you are a traditional hardcore single player type that enjoys intense multiplayer actions, then the PS3 is a good complement to the 360 experience because you have great hardcore games on the PS3 too, such as LBP and resistance.
It comes to taste. I own all three consoles, and I mostly use my 360. PS3 is for streaming movies and upconverting dvds, and Wii is to play with my gf and family and friends.