My 2 cents...
I've owned GH1 & 2 for the PS2, GH3 for 360 and RB for 360.
Rockband blows GH out of the water, it improves upon just about everything.
* I like how the RB guitar has a little lip so when you slide your hand down to the orange button you know when to stop.
* There are tons of little differences that make RB better. You could spend all day just designing your band logo or tattoos for you body.
* I like that they put your band name all over the place, on the side of a bus on the loading screen, and on the big screen behind you when your on stage.
*You can get additional "star power" while your in the middle of using star power, in GH once you initiate star power all star power notes disappear.
* The "tour" theme gives you lots of mini goals instead of just one big one of finishing the game as in GH.
* The graphics look a lot better in RB then GH, I like how they freeze frame each band member with their name right before you play.
* I like how you can bring a failed member back from the dead with star power.
GH for the 360 has dropped the ball in DLC, RB has ripped my wallet open.
My wife got frustrated with GH but she loves singing when I play RB, which means I get to play a whole lot more since she gets to participate.