I liked the guitar from Rock Band 2 better than the Guitar Hero World Tour guitar as it looked nicer and seemed of better quality. I also liked the more robust song list on the Rock Band 2 music store. I don't really care about creating my own songs (Guitar Hero allows for that). And the drums on Rock Band 2 are supposedly a big improvement over the original Rock Band. So we bought Rock Band 2 with the guitar, drums and microphone (though I have no plans to sing).
By the way, there is an awesome this week at ToysRUs for the either the PS3 or XBOX360.
We got it home and we have enjoyed playing the game, having bought additional songs - which is easy to do from within the game. Assembling the drums and guitar is easy, and they look well-made. I have since bought a real guitar strap that has a lot of padding - it really does make a difference.
The training for the drums and guitar are easy to go through, and I have gone through the advanced guitar training again since I felt that I didn't get all of ther tips at first with the Easy setting. The drums take getting used to if you have never played them which is the case for me).
Previewing songs in the music store is easy and straightforward, thought if you run out of points you need to back out and add them through the 360 interface and then go back in. A minor annoyance, but it won't keep you from your music.
Customizing the avatars is also straightforward in Rock Band 2. I haven't done so with Guitar Hero WT so I can't compare. The nice thing is that you can use the GH Guitar in Rock Band 2, so if you already own the game and have another person who wants to play guitar with you, that option is there.
A big peeve that I have is that the quality of the GH guitar seems lacking compared to the RB2 guitar. The RB2 guitar seems more solid and less fragile than the GHWT guitar. So much so that last night my spouse started having serious problems with the GH guitar's strum bar being able to register strumming down properly. We changed the batteries, but the problem is still there. I have contacted customer support about this.
I also felt that the leaps in difficulty between Rock Band 2 were more smooth than in GHWT. With RB2 easy is basically using the first 3 notes and Medium is using the first 4. With GHWT the lowest setting is only using the strum bar and the next difficulty setting was using 4 notes (if I am recalling that correctly).
I like some songs that are on Guitar Hero WT that are not on Rock Band 2's list such as La Bamba. But that is ok. The biggest thing that sold RB2 over Guitar Hero as a whole is the user interface when playing an instrument. In my research, I have tried out Rock Band 2, Guitar Hero World Tour, GH3, and GH Aerosmith (Demo). I noticed somethings that make some variations of Guitar Hero almost unplayable for me.
Simply put, the more ornate the background the notes traverse, the harder it is to discern certain notes either at all or when they are a long note. To me the worst culprit is GH Aerosmith and the easiest to read is RB2. I also noticed that when the notes have an outline, they are easier to see as is the case with RB2 and GH3.
I'll have more to say as time goes on, such as touring, but this is my first pass. If you are looking to decide which one you want I suggest trying out those of people you know so that you can also see any accessories they have. Demos can be useful but I found that playing the games with only a regular controller to be difficult at best. To check out the different instruments, many stores have both types out although ones that I have seen have been treated pretty roughly.