I have PS3 and Xbox 360 and I can tell you I get a lot more use out of my Xbox 360. I bought the PS3 mainly for the Blu-ray which I rarely ever use. The newer releases of the Xbox 360 can go in 1080p (Highest resolution available) without needing to get the Elite version (You just need to buy an HDMI cable). The Xbox 360 also has way more games out. Plus if you going to be playing multiplayer online Xbox Live is 100x better than Playstation online.
If you want to add to your home theater arsenal, PS3 is a great addition. If you like to play games, XBOX360 is where the fun is at.