Long hair, almost always. You can always pull it back or up, French twist, have braids or make a pony tail (ala Sade). I know it's more work, but it's more feminine to me. It's not too often I see the right short cut on the right girl, although it does happen. I think it's a real art to know how to cut hair short - which cut works with which face and which doesn't.
Long hair for all brunettes and some blondes who have a tan as well as lightly tanned women with black hair. I prefer short hair for most dark colored red heads, some blondes with light untanned skin as well as lightly colored women who have black hair. All women with dark skin look great with either short or long hair.
I have had the same question - so I asked as many guys as I could (especially the ones I know) in real life and they ALL said long hair. Which is good on my part because my hair is freakishly long. If you have long hair - keep it long but get it trimmed at least once a month to keep it clean. If you have shorter hair - grow it out. lol
But choose which ever one you feel best in - whether long or short, just keep it to your comfort level.
i love long hair, but short hair is a lot easier :P
and I saw a lot of girls with short hair and that was really sexy and seemed little naughty ;)