I don't prefer Guitar hero to Rock band. I've been playing Rock Band more lately and the only DLC I've ever bought is for RockBand, the point is I'm not biased about them when I discuss them. Even if I hated Guitar Hero I would never say they can't compete.
The problem with people is if they find something they like a little more they start saying they'll never play ____ again. That's pretty stupid because let's analyze what Guitar Hero 3 does, it let's you play songs on a fake guitar just like Rock Band. You obviously bought it to just play the songs on guitar and that's exactly what you got. Now you have the option to buy Guitar Hero IV and play songs with fake drums and and fake guitars. Both games are going to deliver what they promise and there's really no reason not to buy both games since the only reason you are getting either of them is to play more songs you never have before.
Also, Activision just layed down a pretty hard dick slapping to EA today by announcing Jimi Hendrix will have multiple songs with GH IV. http://kotaku.com/5026533/jimi-hendrix-confirmed-for-guitar-hero