Generally long to me, often associated with femininity by our society. I find the very rare case of women who do also look good to me with short hair to be extraordinary though.
They often require very feminine features in spite of their hairstyle, like thinner, elegant neck, narrow face, things of this sort to look very appealing to me, but those that can pull it off tend to be quite stunning.
For most though, I would prefer longer hair, and generally the ones who could pull off short hair beautifully tend to also look just as nice with longer/medium-length hair.
She looks amaaaaaazing she actually looks better with short hair than long hair in my opinion. But she's a rare case I don't really know any other girls that look that look better withnshort hair. There's just something so feminine and elegant and beautiful about the long hair that's hard to match. So don't cut your hair, there's probably over a 90% chance that it won't look that good on you :P.
I've dated women with long hair a couple of times. Otherwise, I only go out with women with short hair and outgoing, bubbly personalities. I have traveled to 45 countries and am very outgoing. I talk to a lot of people in my work. Still, for some reason, I find short hair attractive and long hair pretty much... eh, not interested.