What do you see as the difference? For me, if it's too long to be left out, I'll tuck it in. Then again, if it's short enough to be left out, I'll still tuck it in ... unless it's too short to stay tucked in.
Oh, and I agree on the Henley, but in those, I like both long sleeves and short.
If a shirt is cut in a guyabera style (or a Hawaiian shirt style, or like those "fishing shirts") those are meant to be left untucked, as they are more casual. If it's just a OCBD with short sleeves, those are generally tucked in, although you could leave them untucked.
100% agree on the Henley - I too have them in short and long sleeve.