Since this is what a lot of people will want to know; this game is better then GH:WT for a lot of reasons. Don't get me wrong though, GH is a good game, but its not as polished as RB2 at all.
RB2 offers you far more music (just look at the DLC on the PSN and compare).
Its offers you more options for customising and looking after your band.
It offers more locations to play in.
A more structured "world tour".
More mature music (IMO) compared to GH's MTV stuff.
It's a lot easier to just jump in and play at a party with your friends if you want to because you can just plug another instrument in and go.
It may sound daft, but RB2 just feels like a more polished game which offers better choices, more customisation, more music and just feels more mature.
GH:WT unfortunately just feels like the same old thing but with more instruments. It feels like it was rushed out.
BUT.... the GH:WT instrument are far better then RB's and I would recommend getting those instead as they DO work with RB.