The game depends on on the songs so find the list of both and determine yourself which one you prefer.
Both of them are not out in the UK so I don't know for definite which one is better but I prefer the look of GHWT drum set.
GHWT guitar is seems the better one with the new touch pad on it.
Overall you should buy one and then just buy the other one as an individual game as the both work with each others instruments.
Take note that Rock Band will have over 200 songs out when it comes out through DLC etc but it will cost more, where as GHWT will have a mode where you can make your own music and will be out before Rock Band 2.
I wouldn't bother with Rock Band 1 as the new one will eventually come out, plus the original did have hardware problems with the drums with them ofter breaking a lot.
I'm personally getting GHWT because I believe the drums will be better and on play.com it is quite cheap seeing as though you get a free bass guitar with it.
Hope that helps.