Rock Band. This is why:
Rock Band has:
More songs. Way, way, way more songs For less money.
Better songs.
More accurate charts and more realistic difficulty. (That second part refers to the "slide notes" in the new Guitar Hero games, that make it so that my mother who has a crippled left hand and has never played either game in her life can pass the TTFAF intro. Also the Guitar Hero games have such a wide timing window that the game is artificially easy.)
Guitar Hero has:
Better peripherals.
A few AWESOME songs that I wish Rock Band had. (Free Bird, Crazy Train, TTFAF, Hot for Teacher, etc.)
More "creative" charts--sometimes this is a good thing, sometimes it isn't.