Wiped on 25 October 2008
The Wii version instruments are compatible with Rock Band 2, or so I've heard. I know for sure that for $20 you can get an attachment for the Wii guitar to make it Rock-Bandable (and that works with GH3+RB1 as well).
What with your mates having Wiis, the cheapness of SD cards, Wii jam mode which sounds silly but IGN and others say is awesome, means I'd say Wii version all the way. Plus - and this last point sounds silly - loading times. The Wii version will have none. I played for like a hundred hours on GH3 Wii, and when I tried a 360 version at a mates I felt myself getting so impatient with the loading and saving all over the place - it's so much faster on the Wii!
