Re: GAP AR-10 or POF and why ?
You'll be hard pressed to beat a custom GAP rig versus any factory rifle and I am a proud POF P-308 owner saying this.
First, for the POF...mine has a 20" Rock 5R barrel and with quality match ammo and my reloads, is a solid 3/4MOA rifle and I think I will be able to get better than that with load development. I have shot some 1/2MOA groups with mine, but I won't call it a 1/2MOA rifle without more trigger time and load dev. Every other P-308 I have fired was sub-MOA capable. Mine has been 100% reliable with Magpul LR-20 PMAGs (DPMS and C-Prod mags has issues). I also really like the piston operation of the POF if you are planning on suppressing the rifle. It takes a lot longer to get the rifle filthy dirty when shooting suppressed as compared to the standard DI gas system. Finally, they are excellent rifles and are available (in stock) with different dealers right now.
The GAP should outshine the POF in terms of accuracy. The last AR-10 that George and crew built that I had a chance to shoot was a solid 1/2 MOA rifle with 175gr reloads. If you run a search here on the forum, you should find plenty of experiences with people having excellent results with their GAP AR-10/LR-308 rifles. Reliability will also be just what you'd expect from a GAP rifle. The only downside to getting a GAP AR-10 will be the wait time. I can't tell you exactly how long a wait you are looking at, but I would guess you are looking at a long one. You'll need to check the GAP Leadtime thread or get in touch with GAP personally about a full build.
As for barrel length...that is up to you. 600yds and in...a 16" barreled 308 will get the job done and get it done accurately. The 18" and 20" barrels will give you an edge in terms of increased muzzle velocity which will be advantageous with heavier bullets at extended ranges (generally).
You will be pleased with both the POF or the GAP, but if it were me, and I didn't mind the wait or a DI versus a piston operated rifle, I'd get the GAP custom built to your specs.