Welcome. I too, am considering a .308 AR. I may use the CMMG lower as it takes the HK magazines that I already have dozens of. THe DPMS upper works with this lower, but needs a smal amount of modification to allow for the wider magazine of the HK.
I think you will be unhappy w/the balance of a 20" barrel and a collabsible stock. IMHO, they just look wrong like that.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball