I think it all depends on what you are asking: DI or piston AR? If so, DI all the way. You don't need no stinking piston. The ones I have seen don't do anything the Direct Impingement system does. They are cleaner, but that's about it.
I REALLY like the AK design, which of course is piston operated, and very easy all round, BUT I normall shoot about 196 out of 200 on the USN Rifle qual, and I can't break 165 with an AK. It's the sights, mostly.
Olympic used to make a Pistol upper than ran on Sten mags. Nice setup.