guitar hero III as it's got a lot more challenging songs and better gameplay guitar hero 4 is gonna blow rockband out of the water though as it will have drums as well
nah u can do winner stays on on battle mode or something at guitar hero rock bands only good when u have 4 ppl who go in hissy fits when they can't play at that precise moment :P
Rockband is definitely better then Gh3, because Guitar Hero has only guitar playing against other guitars.
As a response to Shlibub: There are tug-of-war and score dual features in rockband. So, actually there are more interesting battles, like between singer and drummer.