Rock Band IMO. My friends own the game and I love playign with them at thier house. I'm at Expert at Drums and Guitar and I love playing it. If you like the feeling of accomplishment go to guitar hero and pull out your hair over beating Through the Fire and flames. If you like playing more than Accomplishment go to Rock Band. Drums is fun because you are active. Singing is fun if you like to sing of course. if you like Guitar there are several songs in Rock Band that are just amazing (all of the Metallica ones are great, Green Grass and High Tides is pretty good, Epic sounds awesome, Pleasure is very good on guitar, if you like older songs you can go with the 6 pack of Boston songs, if you like humor they added the final song from portal just yesturday). Rock band has alot to offer that Guitar Hero III just doesn't have to offer.
I hope you guys know GH3 was the one made by a different company. Rock Band was made by Harmonix as well as the first two Guitar Heroes. GH3 was made by Activision not Harmonix.
Well rock band is a good party game cuz its fun when you have a lot of people. And you can really only compare the guitar part of rockband to GH3. In my opinion, GH3 is better because it flows a lot better, and the strum bar on the rock band guitar is really uncomfortable.