I have been shooting USPSA for 20+ years and I started shooting PCC division as soon as it was made official.
After shooting both 16 inch and 8-10 inch guns, I much prefer short barrels and a good muzzle brake over longer barrels.
A lighter/shorter barrel swings faster and is easier to move with.
Blowback guns like a AR-9 kick a lot more than I like, even with a brake and playing with ammo/powder and springs. Give me a MPX or a Guard anyday. And they dont eat firing pins like blowback guns.
If you do build an AR-9 of sorts, be sure you get one that uses Glock mags.