comparing the difficulty of Guitar Hero to Rock Band. Their basic conclusion is that "Guitar Hero III hard = Guitar Hero II expert" and "Rock Band Expert = Guitar Hero II hard." They say that Guitar Hero is tough to jump into and Rock Band may be too easy at the medium level for seasoned vets.
Having had significant time with Rock Band on a visit to Harmonix on Tuesday, we can say that when comparing the guitar portion of the game, Rock Band on hard feels close to Guitar Hero II on medium -- except you do have to deal with that pesky orange button, so in essence it feels the same. That doesn't mean that there isn't a challenge in there for the average player, it's just a point of reference for jumping in. Now that's just the guitar, we're not even talking about the beasts of drumming and singing, nor the extreme joy the social interaction of the game provides.