[Ghoulish Delight]

I don't know that there's particularly much gameplay difference, but I recommend Rockband for 3 reasons.

1) CP's brother who works the electronics department at Toys R Us says people are continually returning Guitar Hero drum kits because they keep breaking.

2) Rockband will have Beatles songs at some point (don't know if it will be this version, but even if it's a later version, if you want continuity of staying within the franchise AND Beatles songs, Rockband's your choice)

3) Rockband is developed by Harmonix, who originally developed Guitar Hero and Guitar Hero 2. Guitar Hero 3 and Guitar Hero World Tour are developed by Neversoft (Activision) who retained the rights but broke with Harmonix after disputes over hardware. Thus, while I have played neither enough to have any real grounds for this, I trust Rockband to have the edge in whatever ways the two are creatively/technically distinguished since it's the brilliant folks who conceived the idea to begin with working on it. (though I'm surely biased since my college roommate works for Harmonix).