I own a PS3, and the reason I purchased it is I got a pretty good deal, and all my friends have PS3's. PS3 includes a Blueray player, so that is a big bonus. PS3 has better upgrade ability, you can change the HDD in the PS3 to a larger one if you run out of room, or want to store your entire music/movie library on it. Free online is also very nice.
Xbox has a better controller. The GUI seems better for communicating/playing with friends.
But the PS3 is a pretty robust machine. You could easily use it as a media center as well as a game console.