Long version:
They each have their pros and cons, but overall (since I use the console for gaming and nothing else) the Xbox is an obvious choice for me. Regardless of what anyone says it has all the advantages: huge game library, most 3rd party titles, all the DLC, biggest online community (despite being charged for). All the previous glitches with it have been fixed, and in 1,5 years I've never had any technical difficulties with it. It's also highly compatible with the PC and it's easy to synchronize the two and transfer movies, pictures, music etc to your TV.
Kinda noisy though, and very clumsy and ugly to have in your living room.
I used to own a Wii (it got stolen), but the whole motion-sensor thing you get tired of very fast. And the games feel quite sub-par. But it's very accessible and even people who don't enjoy video games will have fun using it. So it's basically a casual machine, very much worth its price though, and I'll probably buy one when I have the time.
The PS3, I'm kinda ambivalent about. It's a great machine, clearly the best. It supports bluray (which to me is totally uninteresting, but whatever) and doesn't sound like a Boeing each time you turn it on like the 360. One thing though: the graphics are of no difference. This is just some ridiculous misconception between fanboys, that 360 and PS3 graphics differ. No mortal being could perceive a notable difference, and multi-titles are identical. Both systems support HD graphics.
But the PS3 is more expensive, and it has fewer games. Certain downloadable content isn't available on it, and the multiplayer community is way smaller - at times it's even difficult to find players.
Short version:
For a loud, mainstream online gaming box: 360.
For a silent, black box of wealth and dvd-deluxe: PS3.
For gimmicky, casual gaming and funny party-games: Wii.
That's my :2 cents:. But the PS3 or 360 are kinda equal to each other IMO, and it won't matter very much which one you buy. Stick to the one that appeals to you the most, black or white :yinyang:
Hope it was of any help