Here's the bottom line. Rock Band and Guitar Hero World Tour are essentially the same game: 4-player plastic rocking. In the big picture, you are getting the same experience from either game. There are only three distinct factors that determine our winner. They are price, instrument quality, and song selection.
If you don't own any plastic instruments yet, you will need to buy the "Special Edition" of one game, which includes a game, guitar, drums, and a microphone. Note that neither game will get you all 4 instruments, only one of two guitars needed.
The complete Rock Band 2 and Guitar Hero World Tour packages are equally priced at $190. However, the original Rock Band 1 Special Edition is now only $100-$120 at various retailers. This is your best deal to pick up 3/4 of your instruments. You can then pick up Rock Band 2 or Guitar Hero World Tour with just a guitar for another $80-90 and have two games and all four instruments for nearly the same price as just the Special Edition of RB2 or GHWT.
The instruments from all three games work in the others, so no worries about which combination of games you buy.
From the opinion of an "Expert" plastic rocker, the Rock Band 1 guitar is my least favorite of any plastic instrument. The fret buttons are clunky and the strum bar has very little feedback. That being said, I've only played with the GHWT guitar in a store display and never used the RB2 guitar at all. I've used the Guitar Hero 3 guitar since it came out and insist on using it for any songs on Expert. The GHWT guitar is definitely better than the RB1 guitar, but I can't speak to the RB2 version. If you don't plan on mastering the art of the plastic axe, then it doesn't really matter which guitar you get.
The RB1 drums are also considered more loud than the others by the internet community, but that can be solved with an add-on to the drum heads that makes them quieter. Ultimately, you should be playing these games loud enough that the sound of hitting the drums won't be noticed. =P
The GHWT drums are wireless and come with cymbals for two of the buttons that give them a unique feel. Cymbals can be purchased for the RB2 drums, but not the RB1 set.
If you're getting these games for PS3, do your homework. I heard that there's some incompatibility between the games' instruments. Look everything up on Wikipedia.
The Rock Band series has a big leg-up over GHWT in this respect. The songs from RB1 can be imported into RB2 and all downloaded songs work in both games. Rock Band also has many more songs available to download than GHWT. It's worth heading over to Wikipedia and looking up the track lists for both series to make sure there's not some tracks that one game has that you can't live without!
There are also a dozen songs found on both RB2 and GHWT. So be aware of that!
Combining RB1 and RB2 is too good a value to ignore. If you're fine getting only 3 instruments, I recommend getting the RB1 Special Edition and then the RB2 game by itself. It's really worth the extra $30 or so to pick up RB2 with a guitar for maximum party fun. Buying the full RB series should run you about $190 and gets you 162 songs.
If money is no object, then get the GHWT Special Edition, RB2 with a guitar, and rent or borrow the RB1 disc to import that game's songs. Total price of buying everything is around $280 and nets 248 songs. The difference between this plan and the first is about $90, so you're paying for another 86 songs and at nearly $1/song, that's a better price than you'll pay for downloadable songs. It IS excessive to buy all this up front, but I do sign off on the GHWT instruments as the best you can get.
Right now, I own RB1 Special Edition, RB2 (disc) and a GH3 guitar. In the future, I'd like to get GHWT + guitar to replace the RB guitar I don't like and to access the songs of that game.
Which games do you own? What do you think of my assessment?