Long straight hair, as it is easy to keep on model. Meaning that i again have no preference.
Short + unkempt, hm. I never touched my hair with a brush when i had short hair, but that was like a decade ago. I again left it completely alone when i was in the army (i lived though a plot import haircut!). So a 2 year period in my adult life applies, where i had the winning haircut basically every day.
Again, I like all the styles except bald, but I'll go with long straight.
My own hair is a short bob at times and boyish at others (I had it long from the age of 14 to 20, and have never let it grow beyond shoulder length since then), but this is about what I like to see in characters, not real people (really don't give a damn about real people's hairstyles, including mine), so...