I'm partial to RB2 as well, but I really think it's gotta be a choice you make for yourself.
RB was first, so the amount of songs available for it right now is insane. There is a ton of DLC, and like Diablo mentioned, you rent the first game and for $5 you double your tracklist in RB2!! RB also does a much better job of integrating DLC into 'regular,' play, and has a much better song display/sorting interface.
However, being second, GHWT had the chance to innovate with some additional features to their instruments...the additional cymbal on drums,'accent notes,' the touchpad on guitar, etc. I probably have to say that if GHWT was your first experience in the genre, if you go to RB you might feel that a few things are missing. They are minor, but they are there.
Gameplay is pretty similar overall, honestly. RB requires tighter timing with hitting the notes, while GH is much looser while (generally) making you do a little bit more finger gymnastics. Again, this will be a preference thing for you. I tend to think the tighter timing fools you into thinking you're actually the one playing a bit more.
Though cartoony, GHWT tends to have more presentation flourishes. Game versions of Ozzy, Zakk Wylde, etc...more extreme venues. RB is more 'realistic' looking in its presentation, but also more plain.
Consensus seems to be that if you were picking up one set of instruments to play with both, the RB2 pack is better in handling both games without things feeling weird (mostly speaking of drums), but you lose some of the extra functionality of the instruments.
I find the drum charting to be better in RB, the bass better in GHWT, the guitars about equal but maybe slightly better in GHWT, and vocals equal. I think RB plays slightly better in a group, but the GHWT tracks are generally more recognized by non-gamers and thus they get more excited about what tracks are on disc. RB's wealth of DLC goes a long way to balance that out, though. I like RB's online modes much better.
There *do* seem to be fairly widespread issues with GHWTs drum sets. RB1 went through that too, though.
In summary, I think the main thing that differentiates between the two right now is that RB has a buttload more songs, and GHWT has more 'flourishes' that make its instruments a little more fun to play.
I really suggest you check out the track lists, too.
http://en.wikipedia.org/wiki/List_of_songs_in_Rock_Band
http://en.wikipedia.org/wiki/Downloadab ... _Rock_Band
http://en.wikipedia.org/wiki/List_of_so ... World_Tour
I don't know that you can really go wrong. I like and play them both and I'm glad I have both.