People say the wii version was rushed and the gamecube game will be better because it was originally made for the gamecube, but i tend to disagree. The Wii version has had about a year in work so it should be very good by now. Buy the Wii version if you are getting a wii on launch day. If not, buy the gamecube version
Wii, DEFINITELY. The Wii version was most likely in thought for the whole project, but it was decided to actually start making it a year ago. It will NOT lack polish, like some people are saying, but just the opposite; the graphics will be better, the gameplay will be more interesting, and even WiiConnect24 will be in there. Take a look at a screenshot comparison...
GCN -
Wii -
As you can see, the Wii version is definitely more prettier.