Well, my fiance and I own all three, and I get the most use out of the xbox 360. I prefer rpg/adventure type games, (Oblivion, zelda, Fable, Metroid) and they are generally good on the xbox. However, two out of those four are Nintendo games, hence we have the Wii. The Wii is great, but I wouldn't want it as my only console, as the titles for it are very varied in quality. Some of the stuff they come up with I wonder why they ever put it in a game. We don't have many games for it, however the ones we do are generally awesome.
We bought the PS3 a couple of months ago, and my other half gets far more use out of it than me. We have Gran Tourismo on it, and I was very disappointed. Glen enjoys it, but he doesn't play it so much as other games. I don't know if that is just his taste or whether he has got bored with it reasonably quickly.
So for those reasons I ticked Xbox. More excellent games.