I've heard a lot of bad about World Tour, mostly with the character customization and the song list not being so hot, but I haven't played it yet to know if it's true.
I haven't played RB2 yet, but I own RB1 and it's a lot of fun and the downloadable content is great!
Most of the stuff is cross-compatible. One of the big differences is the drum sets. The RB drums have four drum heads and the pedal. The GH set has 3 drum heads, two cymbals and the pedal. The left cymbal is the 2nd most played, and they have five colors in addition to the pedal. From what I've read, the game will adapt, sort of but its a little confusing.
There's some good songs in World Tour, but a lot of crap too, IMO. I wish they would re-release GH III and GH Aerosmith for the full band setup.
My son and I went to the Warriors game the other night. They had GH III set up in one of the little exhibit areas. My son had a blast playing it while I stood in line for food, but the best part was when I came back and stepped in, with other dads watching their kids play and was able to rock it out right along with my boy. He was LOVING having an audience. We're already planning our Driveway concert with a projector for when the weather warms up...