Rock band is very good, but so is GH3. Here's a list of the things I like in both games.
Rock Band:
Camera work while playing, much more realistic and more fun
The color of the notes don't go away when in Overdrive
Create a character, even though all the male models look like girls
The song list
Guitar Hero 3:
The actual gameplay is more technical and more fun to play
Character models look good
The guitar (which I use with Rock Band)
The song list
If they could meld all those things together, we would have the perfect game.
What do you think?