The Stoner design, 5.56 crappy round aside, isn’t going to be beat by a piston driven gun. You can’t connect reciprocating mass to a barrel and have it be as accurate as DGI. Can’t happen.
The question is, which trade off do you want ?
For the regular Soldier’s rifle, do you want reliability or great accuracy ?
I think the obvious answer is reliability, with the piston. Piston accuracy is and will be good enough. They can always call in a sniper rifle, perhaps the DGI M110 automatic sniper rifle, when they need accuracy for distant enemy targets.