Chose either one, they are both cross compatible so its just a matter of which instruments you prefer. Grab the RB2 bundle and all you need to do is buy the GHWT (game only) or vice versa, problem solved.
I've bought both, and I like RB2 better! Here's why:
GH promised, as far back as GH2: "We will provide downloadable songs EVERY week!" Well, when the games (GH2, GH3, GH Aerosmith, etc.) initially came out, they delivered! As time went by, they were lucky to provide downloadable songs once a month!
Rock Band 2 not only provides at least 3-6 downloadable songs every week, but sometimes they will provide a full album also --- Judas Priest, the Cars, Red Hot Chili Peppers, and AC/DC is coming out in about a week! Also, if you register the game, you will be provided with 20 MORE songs when they come out (before Christmas?).....and they're FREE, just for registering!
I have had the RB2 drums for about a week now, and I've ALREADY had to ship the drums back! The rubber over the pads "bubbled up" on 3 of the pads so the game's sensors won't sensor the hits onto the pads! This problem happened with my Rock Band 1 drum kit....but AFTER 6 months of me beating the crap out of them! All of the instruments (if bought as an entire package) have a 60-day warranty. When I went to ship the drum pads back, the lady at UPS said, "Boy, if they have a 2-month warranty, that can't be a good sign!" I tell you, guys, this inconveniencing second set of drums HAD BETTER last longer than a week!
Edit on Christmas Eve: The replacement drums have worked perfectly! I'll be adding the cymbals to my drum set over the holidays, so we'll see how they do!
I think it's worth noting that Harmonix (Rock Band) recently announced they are developing an entire game around Beatles music, in collaboration with McCartney, Starr and Yoko Ono, to be released late 2009. Now they didn't say specifically that it will be a "Rock Band" branded game, but I think it's safe to assume it'll be compatible with the hardware. This is what swayed my vote away from Guitar Hero.
There is cross compatibility for all peripherals for Xbox so it doesn't matter which one you get because the other game will work. For PS3 on the other hand, the GHWT drum set won't work with Rock Band 2 but the guitars are fine. Sadly, everything is completely exclusive for the Wii. In case anyone was wondering, all this info can be found at http://www.joystiq.com/Instrument-Compatibility-Matrix/
I own GH:WT But general consensis is that RB2 is better. I won't argue that but I will say I don't regret my discision one bit.
Not many mentions of this but I like the GH:WT Guitar because of the slider bar. It allows for a more expresive and alternate way of playing. Watch people play guitar on rock band, they all play quite similiar when compared to the a GH:WT guitroller. So unless you're into THAT or drums I'd suggest RB2 =/
The GHWT guitroller is cool, but rock band music is way better for the average person. Unless you are a guitar freak and want to play a bunch of crappy songs with insane guitar parts that you have never heard of, get rock band. If you are a guitar freak and want to play a bunch of insane songs that suck and you have never heard of, then get GHWT.