Rock Band because not only do they have great music (other then maybe the Beatles Version) but they don't keep on releasing the same game Every three months.
i recently bought guitar hero metallica and i just can't forget the rush i got by playing blood and thunder by mastodon and many metallica's music's!truly the best game ever so i say gh
Has to be guitar hero ... though the drums do break easily i find them better than rock band ones. They both have good setlists but what it comes down to for me is:
The beatles vs. Metallica
and to be honest even if i had rockband id sell it to buy GH so i could get Metallica its an amazing game, also i find the graphics better and much prefer the 'gems' scrolling to the lines of rockband which can get confusing and its harder to see which are hammer-ons though that might be because im not used to it
also its generaly easy to use GH controllers for RB but not the reverse so i would advise getting a GH guitar (there allot cheaper if you buy with a game) so you can switch between and get the best of both because some songs that they don't share are unmissable!
all that aside if you don't have one of these games go and buy one with guitar or drums your missing out!