Currently, my "go to" M4 is a CMMG.

It has a 16" M4 profile upper, 1/9 twist, WASP coated.
Magpul MOE stock, Troy rail, GGG vertical foregrip, MATECH USGI BUIS, and an EoTech.

To me, DI vs Piston is an answer to a question few need answered. If you keep your rifle in decent repair/state of cleanliness, then a piston is far from being necessary. They will run dirty longer, and run cooler if firing full auto, but since my M4-gery isn't FA and I've run AR's with 2000+ rds of dirty Russian ammo down the bore between cleanings, I'll stick with DI.