Rock Band is a good game but the problem with it is that even on expert it isn't as hard as GH on hard. Gets boring very fast when you're eventually good.
For guitar, GH3 is definately the best one out there. Rock Band songs can get very boring because of their ease. The reason for that is they had to pick out songs that would be fun for the whole band.
You couldn't do very well with "Cliffs of Dover" on Rock Band.