I'm a different kind of girl [but I'm still into guys!] so I pick more on the straight and long side, just not past the neck or shoulders. Also it depends on the guy's face: generally if he has a small and nice face then shorter hair but a rounder face longer and def straight hair.
I don't like a guys hair (that I'm dating) to be past his shoulders, it's just weird for me. if he's just a friend, it's fine...but I wouldn't want someone I'm dating to have longer hair than me, and I ususally keep mine about shoulder length. Anything shorter than shoulder lenth and taken care of, I'm fine with.
I've a thing for guys with curls, not long tho. Don't know what it is, I have curly hair tho so maybe it has something to do with that. But tbh it's whatever suits the guy best cause I've seen guys with curly hair and it hasn't suited them.
Short straight or wavy hair looks awesome. Actually curly too haha. Colors I guess dark blonde, light or dark brown. Long hair doesn't suit everyone but if you can pull it off then I prefer it straight. and not excessively long.