If I had to choose I'd probably say GH:WT too. The only reservation I have to that is that GH:WT is kind of an unknown entity right now. We know the set list, and we have pictures of the instruments... but with Rock Band 2, we pretty much knew exactly what we were getting because we had Rock Band 1 to look at first. This is Neversoft's first shot at doing the full band thing, so it's hard to say what we're going to get. I'm sure it'll be good, but it makes recommending it a bit harder.
I sure hope it'll be good, I love both Guitar Hero and Rock Band and I want to see both games continue successfully. It's going to have to be good to keep up with Rock Band 2.