If you have the money, RB2 with the GH Drums.
If not, then i'd stick with RB2. You can all import ALL the RB1 songs to RB2 for free. Therefore RB has a way huger library.
The only thing GH really has on RB is the drums. I've heard mixed reviews on the guitars; people insist on the GH guitars being of better quality, but the RB software itself is better than the GH software.
I ended up getting GH:WT and I LOVE the drums. They blow the RB drums away. The deal maker for me was that Gamestop had a GH:WT sale for which the entire band kit was only 90 bucks; 100 dollars less than retail. Otherwise, I probably wouldn't have gotten anything.
Anyways, I found out that the instruments/games aren't compatible w/ each other on Wii. So unless I trade everything in for Xbox stuff, then I'm locked into GH for life now. Stupid hardware lock!