Personally I'd choose a PS3 since
1) You get a blu-ray player
2) Higher resolution so text is a little sharper
3) Space for DLC. I don't know how much DLC GH has since I play rock band. The Wii has very little hard drive space.
4) I *THINK* you can find the game slightly cheaper for the 360 or the PS3
I would also look at
Amazon marketplace before you buy anything off e-bay