Just an update - We bought the Rock Band 2 Special Edition with our Wii and got the extra high hat cymbals and a spare guitar.
For those of you wondering - You do not need a Wii controller for any of the Rock Band 2 instruments. ONLY THE MICROPHONE NEEDS A CONTROLLER! I never found anyone online who could confirm that before buying.
I also picked up the Rock Band AC/DC Track Pack and it has worked fine with the RB2 instruments. No one could confirm that for me either. I have not bought GH as of yet - but we are still having a great time jamming out to the music we have. Of all the GH stuff, the Aerosmith pack is the one I would really like to have. I have heard though that the drums from RB will not work with GH so I may never get it.
Thanks for all the advice everyone and I hope this thread helps someone looking at RB or GH for the Wii.