Also, are Guitar Hero and Rock Band guitar interchangeable on either system?
Not sure exactly what you mean by this. I'll assume you mean, can you use Guitar Hero guitars for Rock Band and Rock Band guitars for Guitar Hero.
The answer is, kinda. Guitar Hero guitars work for Rock Band, but your sleek, shiny new Fender Strat is useless for Guitar Hero.
If you mean, can you use a PS3 guitar for Wii RB/GH and a Wii guitar for PS3 rhythm madness, then I'm a little surprised you're asking, and I'm pretty dang sure the answer is no.
But I think you meant the former.