After playing it I think both GHIII and Rock Band offer huge points on their own...
Guitar Hero III
Pro: Awesome song Selection, Boss Battles are fun multiplayer.
Con: Medium Difficulty can be really hard on newbies.
Rock Band
Pro: Drums, Mic this does have it all!
Con: Even on expert for guitar I think it is dumbed down a bit. Expert in GHIII is not the same as Expert in RB.
My Pick....
GHIII I tend to play more with this than RB. But if I have a crew of 3 or more then RB comes out!