Man this is actually a hard choice for me, especially since the hair must (and sometimes should be) as beautiful as the girl herself. I went for the Long hair (down to the hips) choice because I find it to be the best length while thinking about her.
Personally, I think a chick can have her hair how ever she wants it. I like my hair anywhere between my shoulders and my butt. (That's why I picked about halfway down the back.)