Re: POF Lower...w/which upper???
Look...you can piece an upper together based on a DPMS A4 flattop (the only pattern uppers than the POF is compatible with), but you'll have to use a sleeve to fit the DPMS upper onto the standard-diameter AR pivot pin used on the POF. The fit I have seen on the builds using the POF/DPMS combo have been acceptable, but not exactly tight. The last one I saw was built with an 18" DPMS SASS barrel and consistently shot around MOA with decent ammo (and about 2x that with surplus/147gr ball).
OR...you can just get the complete POF upper in whichever flavor best suits your needs (from 12", 14.5", 16.5" and 20") and know you will have a well-built, accurate (sub-MOA), reliable, piston driven upper. My 20" P-308 shoots 3/4 MOA all day long with good ammo and I have fired some 1/2 MOA groups with it. With load development for the rifle, I am confident that I can get the groups tighter than the 3/4 MOA I have been getting.