I still think that the Rock Band guitars are better. Either way, now's probably the worst time to get any new hardware since Rock Band 2 is close to coming out and GH:WT will feature new guitars.
If you want to play now, just grab something cheap.
The Rock Band guitars may be better, I haven't tried one, but they have one drawback. They aren't compatible with Guitar Hero whereas Guitar Hero guitars are compatible with Rock Band. Now, both sides are saying their guitars will work with each other in future and there was that mystery patch I received when I started when I played GHIII last night, but if you want a guitar that will play everything then go with a GH one.