longer gas systems, suppose to be less wear and tear than a carbine on parts, longer sight radius, less felt recoil(but come on it's an ar, if recoil is a problem go back to shooting bb guns), and you can use a normal ar bayonet on a middy. on the downside mid-lengths have less toys made for them, though more and more makers are putting out midlength rails.

i got both a middy and a carbine, i like the carbine more.