I'd say, in general, long hair is more attractive. It takes a very specific type of face (smaller, thinner, well-angled) to carry off a short haircut. Longer hair is more versatile and can enhance many different types of faces.
Long hair also signifies sexuality and through the ages, it's been a signal to the man of a woman's fertility. So there is a unconscious biologically-based reason that long hair on women is more attractive to men.