As someone who's actually played all three systems, I can give you a bit of advice, but I won't tell you to buy anything specific.
X360) It enters its second year and seems to be getting better. With some AAA titles coming out this holiday, including Gears of War, X360 owners finally have the software to show off their hardware. It's been a bit of a long wait though. With a focus on online gaming over XBLive, Microsoft has done a better job with online integration then either of its competitors.
PS3) Expensive game system, but a cheap Blu-Ray player. If you geek on movies, you might want this one. None of the games in the launch lineup are that compelling though. Also, every single game I've played looks no better, and in some cases worse, than what I've seen on the X360. Technically, it has the ability to look better than the X360, but then again, technically the Saturn could do more than the original Playstation and never got there. Also, if you want this system, you will have to be lucky to get one. Only 400K at launch with (maybe) 700K by the new year. I don't trust that second number though. Personally, if I do end up getting my hands on a launch system, I will most likely ebay it, take my profits, and buy a PS3 again when there are games I want to play on it.
Nintendo) I've decided that I refuse to call this thing a Wii. It's a Nintendo. And its pretty cool. Though a bit gimmicky with some games, the motion sensing is top notch. I see this as a more casual machine than the other two. Graphically, Nintendo's machine is not in the same league as the PS3 or the Wii. Graphics are comparible to high end original Xbox games, which really isn't that bad. I don't see this as my primary gaming system, but I'll definitely own one.
So, those are my thoughts on the systems. Looking over them, I seem more positive on the X360, but then again, its the only system out right now so I've had the most time with it.
*Marketing Schill Moment* If you do pick up a Wii, make sure to get Elebits! *Moment Ended*
Yes, I am marketing scum.
- Colin