As an owner of both the 360 and Wii, I'd definitely go with the 360 version. DLC is less of a hassle because of the Hard Drive, if you plug the systems into an HDTV you'll actually get some benefit from the higher resolution display (it makes it easier on the eyes to track all the instruments and lyrics), and in the event you get a full surround sound setup hooked up you'll get better audio fidelity out of the 360 version than the Wii version.
I actually held out on purchasing any Guitar Hero / Rock Band games when I was a Wii only owner (I bought a Wii on the morning of the North American Nov '06 launch and didn't get a 360 until May '08). While the games are certainly still a blast on the Wii, if you have a Wii and either a PS3 or 360, I'd say (without reservation) to go with the HD console version.
The only downside is, of course, that you don't have free online play on the 360.







