I used to say Guitar Hero, because I loved Guitar Hero 2 and especially 3. I still play it too, but as of recent I'd have to say I prefer Rock Band. GH4 was terrible, and with all the games they are releasing it's just driving the series into the ground. I also prefer the overall look and feel of Rock Band over Guitar Hero. It seems more... modern and clean, I guess, and you actually feel as if you're playing in a big stadium more than you would if you were playing GH. Rock Bands songs and DLC are also more to my style. They, in my opinion, are more of the Rock genre than those of GH.
One thing that I think Guitar Hero has on Rock Band though, is challenging songs. I wasn't challenged by ANYTHING on Rock Band 2. Guitar Hero definitely does have the harder songs.
But yeah, Rock Band for me.