personally I think Guitar Hero is better (or at least harder, had 5* on all rockband songs on expert but can't get the "222,222 on satch boogie on lead guitar" achievement) for guitar tracks because it has had a lot more experience than rockband and the vocals are better (dont know why on this one, they just seem easier) Rockband is better for band play, drums and bass (Guitar Hero has very easy bass)
i'm with jess. Rockband is way better. Just watching guitar hero is painful. the animations arent that great, plus, guitar hero destroys a lot of great songs using cover up bands. Rock Band has realistic animation of a band, and never uses cover up bands. The only half decent guitar hero is Guitar Hero Mettalica.
I think Guitar Hero is much better, but I do like how there are a variety of instruments to play in Rockband. Also, Guitar Hero has a lot more games out, which gives way more variety than Rockband.
Guitar Hero is has more insane solos and harder one player mode. But Rock Band has a better multiplayer mode to it. Who could forgot GH has Jordon and TTFAF.