We bought the Wii Rock Band. It came with drum set (with el cheapo wooden drum sticks for the decent and functional, cymbal-less drum set) and the Rock Band 1 black and white, small plastic guitar.
Then we got the Rock Band 2 guitar.
Both Rock Band 1 and 2 guitars work well, though the Rock Band 1 guitar got tired of me strumming from low2up and the strum bar would only work up2low.
The RB2 guitar is much prettier with the fake wood grain paneling. The RB2 guitar is nice in giving a small feedback 'click' when pressing on the different colored tone bars on the neck of the guitar.
I use both guitars and have come to have no real preference however. As an adult RB1 and RB2 are about the only Wii games I will play, having little liking for the Wii controllers (I much prefer the Nintendo GameCube type controllers).
I only wish more songs were available, more easily for RB1 and RB2.
And that an adult sized RB1/2 wireless guitar was available from Nintendo. From Nintendo as I've found in general to avoid 3rd party controllers, particularly the ones that sell for less than the Nintendo units.
Buy Wii Rock Band 2 Standalone Guitar!