This is something I have quite a bit of experience with. I have a GAP-10 with a 22” Bartlein barrel, and an Aero with an 18” Ballistic Advantage barrel. I use the GAP for matches and have taken it to 1200-1400 SEVERAL times without problems. The 18” feels MUCH lighter and more maneuverable. Not even a comparison, the 18” is much easier to shoot, move and re-establish.
I would go with a 20” if I were in your situation. The 20” will give you the velocity to get out as far as you likely want, while still being able maneuver it.
Since I have an 18” and a 22”, I don’t think I’ll necessarily run out and buy a 20, but I think it would be a great “do it all”.
I know Jack Leuba from KAC is running a 6.5CM in a KAC gun and he has stated that barrel length for length, the 6.5CM just flat out performs the .308. So, a 6.5CM in a 16”, 18”, or 20” will outdo a comparable .308. I see the 16-18”
6.5CM being a BIG thing very soon.