I think Gh4 is better,
I have rock band 2 as well but prefer the notes on GH4 as well as the visual style of the game.
Rock band is fun but the graphics are poor and it seems alittle more tedious.
I updated and its alright, but I have no idea where anything is and its seems messy, it takes ages to go through freinds lists etc....
But some features have sped up and become much better, ;)