As a practical matter, there is no huge difference. The rifle length 18" will usually feel smoother when shooting and there is less barrel poking out of the handguards to brand yourself with if you are doing a lot of shooting.
Either one will work just fine. For the record, I once faced a similar decision. I needed a new barrel. I wanted a midlength; but didn't have the money for my new go-faster carbon fiber barrel and a new rail system. I ordered the carbine length and they shipped a midlength instead. Taking this as an omen, I kept itand bought a new rail. Now after several thousand rounds, I can say that it would have been better and smarter for me to have spentthe extra money on training rather than minor tweaks in gear.
Having said that, I love my midlengths
