You say that Guitar Hero is more straightforward, but that really depends on which Guitar Hero you go for. Guitar Hero: World Tour and Guitare Hero: Metallica both have full band play just like Rock Band, and all future Guitar Hero games will as well. In fact, in the upcoming Guitar Hero 5, it has a party mode where you start a playlist going and people hop in--on whatever instrument they want, regardless of who is playing what at the time (so you could have an all-drum band)--at their convenience. That game play mode alone has "public program" written all over it. Do keep in mind that GH5 isn't due until the fall.
Rock Band does have a broader sampling of artists. One thing I like about Guitar Hero is that, despite its party game aesthetics, it is made moreso with the music enthusiast in mind. Rock Band tends to stick more with songs that everybody is familiar with and current radio hits while Guitar Hero highlights more critically-acclaimed material with occassional popular sample.
Some other things to consider: Rock Band 2 has a "no fail" mode where essentially you can't lose. Guitar Hero is glad to tell you that you failed. You can toggle "no fail" mode on or off in Rock Band 2, but some gamers are offended by its mere existence (oft said line: "Halo 3 doesn't have a 'no fail' mode.") So if you want to do casual, fun, open gaming nights fire up Rock Band 2 and turn on "no fail." For competitions, go for Guitar Hero.
Personally, I prefer Guitar Hero. It has more personality and feels more like a real game; and if you get Guitar Hero: World Tour then you can still have full band play. Rock Band is fun without a doubt, but there's just something about it that feels cold and empty. I use Rock Band 2 for my programs, because at the time I purchased it Guitar Hero: World Tour was having problems with their instruments. Activision has since remedied that, so either game is good to go.