Hmm, hard to pick one specific, especially without the images. But I'd have to say I prefer either large/v-large or small, pert and jiggly. Not a big fan of "medium" or "perfect" and I'm on the larger side myself anyway.
As for guys' butts, I'd have to say tight and toned (something you could slap as you walk past and meet resistance on). Otherwise a small, but round butt that I like to refer to as "eskimo butt".
