Mii Freestyle mode is actually an exclusive game mode in which players can create music on the fly using Guitar Hero guitars and drums -- or the Wiimote and Nunchuk as air drums, Wii Music-style, if they want to for some reason. The instruments lay down predetermined notes or sequences onto the familiar Guitar Hero note chart according to the style of music chosen. It's sort of a jamming version of the Music Studio found in the game. Vicarious Visions' David Nathanielsz calls the mode "a helpful bridge between the core game and the Music Studio in Guitar Hero: World Tour for Wii owners."
The Wii World Tour, then, has all of the features of the other versions, plus a new exclusive mode. The only thing the other systems have on it now is graphics, which matter very little in a music game. Barring accidental monaural sound output or something, the Wii version will now be the superior version. Take that, Rock Band!