on girls, long hair is very pretty but hard to take care of. If its straighter, go for a longer cut, if it's curly go for a shorter cut. Long hair is hotter in the summer but helps more in the winter.
for guys, I'd say if it's too short, it doesn't look too attractive- i personally like guys with some actual hair on their head, a medium length (nothing that drags down to the shoulders, but isn't less than an inch). It has to be actually neatly kept to look good, though. if your hair is curly id go with shorter, if it's straighter, go for a flatter cut, but if its wavy, go for a longer cut.