I think hair makes a big difference with someone's appearance. It definitely depends on the guy. On some facial structures, very short hair is the most flattering. Some guys need some hair to make them look their best. When a guy's hair starts getting too long is when I find it the most unflattering, for example so long they can put it in a pony tail, or if it makes them look like a girl.
If you are really interested in which looks better go to someone that knows a lot about hair and show them a picture of you with both and ask them which they think is the most flattering.
I asked a question on this site about whether my hair looked better longer or shorter and I actually got a ton of responses. You should try to do the same thing.