Long hair is good, but only if it's reasonably long and healthy.
Most girls who grow their hair past their midback have nothing but dead, stringy, split ends and that looks bad and unkempt.
Generally speaking shoulder to midback is preferred because it suits almost every girl. Short generally is unflattering an too long is weighed down and dull/blah. Most girls can't rock short or long hair.
I say just make sure to trim the ends. Too many girls with long hair have nasty looking hair. It's beautiful when it's healthy, especially if it black. But I'm biased since I absolutely love black hair
In general most guys prefer long hair 100% of the time. I personally think soulder length or longer hair ALWAYS looks better. HOWEVER, her reasoning for keeping it short CAN make it better. A girl who has her hair short because she THINKS it looks better is just plain delusional. But a girl who cuts her hair short because she donated it to locks of love is EXTREMELY attractive for that reason alone. Also I like the idea of a practically minded girl so if a girl chooses to keep her hair short because long her gets in the way or is too much trouble, I can perfectly understand and appreciate that and the practicality of it.