Gas.
Why pay more for something :
- Not proven yet
- with more parts, hence more prone to failure.
- totally overpriced
- non conventional, with specific parts made according to each manufacturer... Good luck for maintenance in case sg goes wrong.
I run a 1990's Colt, and after thousands of shots... Never had
any issue, even after I got enthusiastic and shot half a crate in an afternoon.
If you clean the rifle regularly, you won't have any problem. Nobody does, actually, except marketing guys that are pushing hard their piston
Cleaning a gas system is really simple and quick. Anyway, gas goes so fast in the tube it does not clog it... Stop the urban legend. Just give some love to your carrier key and you won't have any problem.
DDM4 Vx are, IMHO, best AR option today on the market. Under priced considering the level of manufacturing. My second choice would be a DPMS, if on an budget. 3rd will be a Norc, if on a tighter budget.
I have considered piston a while, drooled on Barrett's REC7 - supposedly the best piston on the market but :
- price tag is ridiculous.
- nothing outstanding in 5.56. Rifle is great in its 6.8 SPC version. This is a hard to find ammo, with a very specific range of application that nobody here should care about.
My $ 0.02 is, if you shoot with ammo of reasonable quality, a gas AR is by far the most sensible choice. Not the hypest, but sensible
And if you want to shoot cheaper ammo, get a Norc for $600! They work as good as the aforementioned brands, they just lack the polished features and the nice coating. But they work...
Bottom line is : any AR system need to be cleaned after heavy use to work well. As any other gun.
Hope it helps.