Rock Band vs. Guitar Hero?

I want to buy one of these games, primarily to play with my wife at home. It seems like there are so many different versions that I don’t know where to start. Which game and which version is best suited for two players? Which is the better overall game? Do they come with the instrument(s), or do I have to buy them seperately, and if so, which is the best value? I’m talking about the xbox 360 version here, if it matters. Any input is very much appreciated.