I think it's an interesting one, because they are so different in looks so a lot of people will like one more than the other, at least to some degree. Taste is a weird thing though, it's different for everyone. From what I've seen, I think people seem to like Mel more than Sarah (among those being vocal about it).
I'm massively the other way – Sarah is cute, but Mel doesn't do it for me at all. I think I remember that she has a pretty good body, but I don't like her hair or ink. But then I like Zoey, and although I don't like the ink there either (just a preference against big pieces, particularly in certain locations), she looks good enough overall that it doesn't work against her overall ranking (or not much, at least).