Having played most recent GHs and RBs, Rock Band is a lot better (RB2 destroys GHWT completely). Plus, RB Beatles is coming, that's a huge plus

It looks like you may want to get the full World Tour kit, and then Rock Band on its own, as the Guitar Hero instruments are compatible with pretty much everything.

Yes.

I just got Guitar Hero WT for my wii, I decided to get it because the guitar works with RB2, so I'll get RB2 later which is the better game.

RB2 don´t support Activision greediness.

Guitar Hero: World Tour

Guitar Hero 3.

I agree with this. I first got GH3 then GHWT so I now have the entire band :). 

I'm writing this from a Wii perspective.

GH3 good game, but not a band game. Fun for playing buy yourself.

GHWT is a good game the instruments are all compatible with RB2. Though I wasn't all that impressed with the songs on the track list. There is nothing wrong with them, but I liked GH3 better. It also has a good character creator. Song I feel are better fretted though and the extra playing styles(slider and multi hammer ons) are great.

RB2 is an overall better gaming expereince. Tour mode is really fun. The game has Band management which is a really awesome bonus. You can setup secondary band members if other players are not available. This is great because your band doesn't feel like some random group of nobodies(GHWT has no band management). RB2 has a better starting DLC list, but GH4 series is working on catching up(though I have never seen a song I want). Menu system is also better orginized. The character creator however is greatly inferior, but passable

GHWT is an excellent game if all you want is the straight guitar gameplay. Rockband 2 is a better band experience. So if it's just you you might want to just get WT, but if you are often going to play with friends or online band your probably better off with RB2.

Ok finally. Get GHTW for the instruments and then buy RB2 on it's own. This is what I did and I'm really happy with it. I don't much care for the RB2 instruments from what I have tried of them. Also the Les Paul is compatible with RB2, so while I thought I wouldn't be able to use my GH3 LP with RB2 I was pleasently suprised it works. So I still have the full band. Since it's for me and the family more of my family is happy with it. Having the choices has also been really nice.

RB2 for me, but i just prefer it's playlist over World Tour's

Guitar Hero World Tour guitars and drums work with all Guitar Hero games and Rock Band 2, but not Rock Band 1.

Rock Band 2 guitars and drums only work with Rock Band 1 and 2, but no Guitar Hero games.

I don't know what's gonna happen with The Beatles though. Eventually I'm gonna buy the Beatles, whichever instruments work with the Beatles, and then whichever games work with those instruments.