I think it depends on what kind of gaming you are looking for.
For starters, I still think the PS3 is way overpriced, and the library of titles doesn't warrant consideration from me.
That leaves the Wii or XBOX.
The Wii is tons of fun, but the entertainment value is more family oriented, more for softcore gaming. When you get past all the sports games requiring you to throw your arm around, the big library titles are more of the same old Nintendo Mario/Zelda titles. I'm not knocking the Wii, I love it and am buying one in the next week for Christmas... Definitely the better option for family members to get involved. The Wii also has tons of 8bit/16bit games on the virtual console from older systems too...
That said, the Xbox has more of the current hardcore gaming, type games. I'm an old school gamer, and I'm sorry, there's just too many damn buttons on these controllers.

But if you're a younger guy that loves all these 3D titles where you blow shit up nonstop and the whole universe of the game comes down on your head, then the XBOX is probably for you.