I just read an article about that today.
Here is a link to "
Rock Band versus Guitar Hero" on MSN, for as long as the link lasts. It goes over pretty much all the pros and cons I can think of.
I don't own either and have only played Guitar Hero, so I can't really help you. Based on the article, one point I came away with was that (hardware and play lists aside) Rock band is more about playing as a group and having fun, while Guitar Hero is about challenging yourself in single player. [With Rock Band you can unlock all the songs from the start and play in 'no fail' mode so you can use the game as a fun party game without worrying about one guy ruining everyone's fun just because they've never played before.]
I enjoy Guitar Hero a lot and have an illogical bais in favor of it, especially now that they seem to have caught up with the new instruments and all... but I'd definitely consider either one after reading about both side by side. I wish I had a chance of getting either one for Christmas. Once you get through the painful learning stages, it's so much fun to rock out!
[Edit: I don't know how possible it is, but you might want to consider convincing the gift givers to look into buying each instrument seperately (RB guitar, GH drums, etc.) and
both games. Haha. Like I said, I don't know if that's even possible but I imagine picking and choosing from the best of both worlds could turn out pretty nicely, since they're both cross compatible now. The exception being that if you like GH better and want to get an older game, the RB guitar wouldn't work for it. Only GH equipment with a RB standalone disk would be completely backwards compatible, I think.]