I don't know, since I have Rock Band, I tend to keep track of GH DLC, and I always see a constant stream coming out. If I could make any generalization about either one, I'd say that GH DLC tends towards more mainstream while RB DLC tends towards more underground. Of course, that's a massive blanket statement, as each game has a mix of both. But it seems like Harmonix prides itself on its "music nerdiness", and the RB track list reflects that to a certain degree.
I'm curious to see what Harmonix does in response to the GH music editor. I'm sure they'll go out of their way to one-up it in every way possible, and maybe even gear it more towards "serious" musicians.