Rock Band is boring as hell; you play the same songs over and over again, and frankly they're not that great.
Rock Band's guitar is also really. really. uncomfortable in comparison to Guitar Hero's. Rock Band is also a major eyesore visually (not that GH is much better, but it IS still better visually). Also, GH isn't about MTV trying to cash in on something popular. Modern MTV generation is so. commercial.
Anyway, seriously- Rock Band was really boring. I still play GH2 and GH3 a ton even though I've beaten both, but I still enjoy playing the songs again... because I'm not forced to play the same songs over and over again to beat it like I am in Rock Band