DI is fine for civilian use.
Theoretically the DI is more accurate as its true free floating.
Reality is that the accuracy of the AR is way overhyped. With match grade rounds the DI would likely be better, but average ammo, no.
The army found out how to keep the DI working in the worst environment, not a great way to go but when you are stuck with it, you either make it work or you change it.
You will note that the Marines are doing an underhanded shift to the Piston system MK27, and they went with piston on the HIK416 and 417.
Keep in mind, those guns were deigned with it from ground up and will fit into an AR lower (sans some of the intentional fit offset by the ones make in Germany).
An upper AR with a piston that has been modified is not the best setup, but again its not going to see the kind of engagement the military does and probably works fine for civilian use if its made by one of the better AR mfgs (RRA, Stag if they made one, S&W, Rugger, i.e. companies that will be there and better yet, the ones that stand behind their products)
Hard to beat the simplicity of the DI.
And keep in mind, most pistons are actually gas tappet/short stroke piston systems. They are not the long stoke direct drive of the AK and Garand actions.
Robinson Arms XCR is an example of a great system that does an extremely good job of bring the AK system fully up to date and the best ergonomics in controls of any carbine I have shot or handled, but their customer support is always on life support. They would be an example of who not to buy from as they could be gone any day and stagger along.
If I was going to switch he military to a piston, thats what I would do, buy out RA and take over their pattens (send Alex to an Island someplace with a machine shop and let him play by himself, grin)