i would say it's about how long you can concentrate on writing, and how hard you can concentrate on filming...
if you don't want to spend a few months on it, then write a short and film it...
if you can manage to roll out a full project takings many months, write a feature.
In my eyes, a short is for showing your talent.
In my eyes, a feature, is for showing your talent and a chance at making money on doing it.
or said another way, it's almost impossible to sell a short, but it's possible to sell a feature.