If your more into the Song Creator for GH and prefer the GH instruments, then you probably want to get GH:WT. I personally prefer the Rock Band gear better, but I think I may be in the minority.
And there really is no way of saying which is better at this point because the GH instruments aren't out yet and the Rock Band 2 stuff hasn't been out long enough to say whether they are a higher quality build than the previous instruments.