I've been kind of wondering this for a while. I know Rock Band has a big fan base, and I've heard that GH had one too. But I've noticed a lot of hostility towards Guitar Hero. But to be honest, I like both franchises. I like Rock Band's overall game-play, as the note charts seem to be much better constructed than Guitar Hero, but honestly I like almost everything else about Guitar Hero more. Guitar Hero usually has more music that appeals to me(Except Band Hero. It's set list sucked major league!). More Indie music, more poppy-fast paced music, less slow music, less folk music, and usually less mainstream (Except Band Hero which was almost nothing but Mainstream). Plus I prefer the more colorful graphics and some of the stages in the Guitar Hero series too. (I pay LITTLE attention to those KFC ads). While Rock Band does have some cool stages, I really hate the ones that fade to almost entire black, as well as the stages with huge blaring lights strewn all over the place.
I often go back to both series from time to time. I usually play Guitar Hero when I want to listen to a specific song or practice drumming, but I play Rock Band when I just want to rock out (and to a lesser extent for a specific song). Also I have a lot of fun singing Painkiller in Rock Band 2, trying to imitate that screeching voice.