I'm torn too, but I'm also considering buying the 360 version so I can use the GH3 controller as a bass for Rock Band.
The selling point about the Wii version: online play. I know they're committed to getting online vs. and co-op for all three next-gen systems, so if you want to get your global rock on, go for the wii. If you're happy with rocking out with friends at home, though, might as well save some money and just get the PS2 version.