Personally, I like the setup in Rock Band 2 more. The song list is more organized and you can transfer over any songs you have downloaded from the first Rock Band.
I have both games and I find that the weird secondary bar slider thing on the World Tour guitar controller is a bit off and almost always not worth it. The idea is good, but better luck next time it seems.
For the DLC, I find that Rock Band 2 has a much better selection, while World Tour doesn't quite have a big enough selection yet, and most of the songs are either by Metallica or Oasis, or they are random international songs that I've never heard of. Again though, just google the DLC lists for the two games if you want to compare them.
It's just better overall, whether it's songs or gameplay. The multi-player is awesome on GH:WT and one of the really sick things about Guitar Hero is that you can go to their site and track your statistics. Very cool.