Yeah, I also vote for the GH2 guitar on the 360. It's the only one of my 4 guitars that actually works. I have the GHIII Les Paul (drops notes), GHWT guitar (drops notes) and RB1 guitar (awful strum bar, overdrive hard to deploy).
I haven't really tried the RB2 guitar, though. The strum bar seems the same, but it's probably more reliable than the GHIII/GHWT/RB1 guitars at least.
All the guitars currently released for the 360 work on RB1 and RB2 and all the Track Packs and other offshoots. No reason to think that's going to change. However, the Pearl Jam project may just be DLC, not necessarily a disc.