You see this is what is frustrating about this debate...that shouldn't even be a debate.
Harmonix created the music game with toy instrument genre, so they deserve a lot of respect.
But you know what, Guitar Hero 2 and Guitar Hero 3 to me are both great and fine games. Neversoft got handed the task of replacing Harmonix...here is something, Neversoft probably requested the game, or at least hand picked the team of designers that loved music games and music to head its Guitar Hero 3 team.
Yes, Harmonix decided to expand the series with adding new instruments with Rock Band, and it was a huge success, which is why Guitar Hero 4 is following suit. But that doesn't mean GH4 isn't going to be good...or that the designers can't love music and desire to create the best music game possible.
To illustrate this, GH4 is actually doing several things to make their game better than Rock Band. Character Creator, Music Creator, tons of songs already on the disc, broadening the music types for GH4 instead of just metal, Better Drum set that is more like a real drum set.
It seems to me Neversoft is using what Harmonix learned and pushing forward to better the product, and Rock Band 2 is just a me too product as a reaction to GH4. Sturdier Instruments? They could have just released those and be done with it. Does this mean I think Harmonix is coping out, no.
What I think is going on is Activision sees Guitar Hero as a huge seller and wanted to pump out more games via Aerosmith to make more money...but they hadn't built the infrastructure for sharing downloaded content and such. However, I believe that with Guitar Hero 4 that will be built and in place.