Soundtrack is all up to your personal taste...check Wikipedia, they have tracklistings for every game. I'd say Guitar Hero: MetallicA has the best setlist, but obviously, a lot of people will disagree with me
I personally don't really prefer one over the other, though I've been playing a lot more Rock Band lately, since I'm digging the drums.
Though if you're considering the Wii ones, the Rock Band controllers will not work in any of the Guitar Hero games (except for the microphone). And the first Rock Band for Wii is a pretty lousy port.
I don't know if the new GH5 or Van Halen supports Rock Band controllers or not.
Edit: Forgot to mention, Red Octane (Guitar Hero) makes much better controllers. I don't know who makes the Rock Band ones, but they're pretty darn fragile.
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