My opinion: go for the Wii, when it comes out on December the 8th of this year.
I mean, the original XBox and the PS2 were very similar: most of the major titles that were available on the PS2 were also for sale on the XBox.
But the Wii will have new, fresh and exciting games. Also, it will be considerably cheaper than either the 360 or the PS3. It seems to be following the same philosophy as the DS: look at how many people who don't usually play games, have gotten into gaming, because of the DS. I was in my local games store recently, and I saw two people who would otherwise look like non-gamers to me, go into thr DS section, to pick up games. You can play Gamecube games on the Wii, aswell.