the difference between Mid and Carbine is not specific to any brand of rifles or uppers. there are two major differences.
the first is personal preference/future plans for the upper. a carbine has a shorter hand guard, so where you prefer to hold the weapon is the deciding factor. a mid hand guard is 2 inches longer than a carbine hand guard. with an AA upper that also limits the rail length you can use in the future. the longest carbine rail you can use is a 7 inch rail with a gas block cut out (9+inches total) and a mid will be a 9 inch rail with a gas block cutout (11 inches total)
the functional difference is the felt recoil impulse. the longer the gas system, the more gas will be burnt off, and the softer the felt recoil impulse will be. rifle is softer than mid, is softer than carbine, is softer than pistol.