Last year, Rock Band beat out GH III due to innovations. This time, while GH III is awesome, RB 2 has these cool features:
1. More songs - you can import the RB 1 songs into the current RB 2 game - along with your RB1 downloads.
2. More content - RB 2 has No Doubt, Roy Orbison, AC-DC and several other acts lined up. I haven't heard a peep out of the GH camp.
3. MTV. Yeah, MTV has a major stake in RB, and their support is great for any game.
4. Less to invest. Let's say you have RB 1. Don't wanna buy wireless instruments? Buy the game - and use your old instruments. If you want the full flavor with WT, you have to buy instruments.
In the end, RB 2, FTW.