You asked the right person
1) GH or Rock Band: Hm... difficult choice. I personally love GH3's songs but then again, Rock Band has INSANE amounts of DLC. So I think Rock Band wins it.
2)Controllers are not easily available. I'd suggest ordering the whole Rock Band bundle from some website. It should be noted that in any case, Guitar Hero guitar controllers are 10 bazillion times better than the Rock Band guitar, or as it's called, the craptar.
3) Check prices online, because that's where you're buying them from.
4)Guitar Hero guitars work for Rock Band, not the other way around. Drums of both work for both.