I think the Wii is worth buying just for this game. The Legend of Zelda has the best storylines and characters of any game around, it even has the best music. I would put money on this being almost on a par with OoT.
Twilight Princess has minor graphical improvements on the Wii apparently, and the control scheme is different. The only difference in gameplay in GC and Wii versions is that the Wii version has Navi in it. It has been referred to as Navi by Eonimu (???) Aonuma but I'm not sure whether there will be a cutscene in which Navi believes that this Link is the Link she lost years ago, whether this comes before Ocarina, or whether she is there for help with the Wii controls.