Rock Band 2, to its credit, has the backing of MTV and EA. They can sign tons of acts - like the new No Doubt content. GH did good with III but Rock Band stepped up and showed the world folks weren't scared of paying $200 for a virtual band.
Even with the song creator, GH doesn't offer the value that RB does. Shoot, you can even import your RB songs into RB2. That's so full of win.