I want the 360 first, just because I am a fan of the original XBox and of the Live service, and I'll get it once I can afford it. I'll be thrilled if they lower the price to $300, I've seen rumors, but especially after the PS3's ridicolous price, I have no complaints with $400
BUT, I do want a Wii at launch! It'll be affordable, fun to play, have a lot of different and fun stuff to do, and the Virtual Console. With it being affordable, I'm gonna try and get this system on launch day for sure.