I meant.. the actual system..
Do you use the Direct Impingement or Gas Piston system?
Im trying decide which I would prefer.
Then why did you say "I'm preparing to start my first build which will be a midlength"
That of course implies DI gas system so you lead us down the carbine length, midlength, intermediate length, rifle length pathway.
Anyway, my first answer stands, I'd go with a 16" barrel using a midlength direct impingment gas system. For the same reasons kraigwy and creeper said. And for cost and simplicity over piston.