Guitar Hero: World Tour will be a fairly major departure from the previous
Guitar Hero games for RedOctane, so don't judge it by the performance of previous
Guitar Hero games. Since the success of
Rock Band and the way they decided to use DLC, it's entirely possible that RedOctane will do the same thing for
Guitar Hero: World Tour.
That being said; I have
Rock Band and
Rock Band 2, and I love them. I'm going to buy
Guitar Hero: World Tour just so that I can play some of the songs on there that I have been wanting for a *long* time.
Guitar Hero: World Tour (at least for the Xbox 360 and PS3) will be able to use
Rock Band instruments as well, so I'd imagine they'll make a game-only package available at release, or not long after.
If you have to only have one, I'd say
Rock Band 2. Or you can do like I and many others will be doing, and buy both, and switch off depending on what song lists you're in the mood for today.
So.
Rock Band 2. For now.

HTH.