I can't really offer an unbiased straightforward answer, as I'm biased towards Guitar Hero 5, so I'll give my opinions on what both have, etc.
RockBand Beatles
-Very fun if you LOVE the beatles
-Could be pretty darn easy as, though they were EXTREMELY revolutionary, there aren't insanely brutal type of songs
-Vocal harmony system looks fun (2-3 singers have to sing at a certain pitch)
Guitarhero 5
-More than one guitarist (not sure about drums and stuff, but I could only imagine so)
-All songs unlocked at beginning
-Can jump in and out of a song at any time
I'm pretty sure your Rockband Drums, mic, and even guitar will work with guitar hero 5, so if you're worried about that, no need to be. Both are very party based, so if you're looking for a game to have when you have friends over, it's all about personal preference really. If you get the Beatles rockband, I'm always drummer, as I feel guitar COULD (I still need to try it out though) be too easy as most of it is just chord progression.
You might complain about how there is no point in playing GH5's story as all songs are unlocked. Though that may be true, you KNOW you hate working for crap like this. Having to beat hard songs, etc. It gets a sense of accomplishment with hard work, but it's a video game. It's just for entertainment. Guitar Hero 5 will have more variety, but with it, there are definitely less classics than Beatles Rock band as it's ALL classic stuff.
Oh, and Beatles Rockband has a lot of emphasis on the singing because of the vocal harmony, and out of the four of us, no one really enjoys singing.
But whatever you do...
DON'T GET THE MOTHER%^&*ING DJ HERO!!!!!!!!!!