I have built a handful of ARs and here is the thing, almost all of the really big guys are making the same product made using the same small parts and forgings from one of 3 Aluminum forges. That's to say if you know what makes makers marks to look for, the name in big letters doesn't matter, and finding the best value in the bells and whistles and differences in fit and finish can be very challenging, but any name brand rifle/pistol will flat out run.
I wouldn't buy any of them new, because I know I'm going to change out half of it regardless. It's one of the very easiest platforms to put together from scratch and I STRONGLY urge anyone to do so. Nothing will give you confidence in your tools like making them yourself.
As far as DI vs piston, if you look up the military tests the DI colt M4s were sand blasted continuously for 25 hours and then fired. Collective 60k rounds. Those DI rifles experienced a less then 2% failure to function rate under those conditions and none of those were difficult to return to action. That's incredible performance under the conditions.
Now, the DI system IS inherently more accurate, however shot to shot accuracy of piston DMR and precision styled rifles is still sub MOA and with FAR less heat and grime in the action, and piston rifles (but not AR), had half the failure rate or less in the same tests, stupid reliability. There are a fair few piston rifles on the market and conversion kits too. Now a good military contractor (Colt or FN) DI rifle will run you $1100 or so, and the contractor (HK) piston rifles run you around $3000. That's not worth it to me, not even close.
On the other hand, the Ruger piston SR series is around $1500, and a lot of rifle for the price, but the Adams conversion kit is considered to be the best by the most and tics all of my boxes for features: forward gas ejection, adjustable gas block, and short stroke piston/transfer bar system (like an sks/svd) runs about $200 as an upgrade to my $3-500 pistol piece meal builds, or $500 Ruger rifle/pistols. Now THAT is my speed.
So, I urge you to build either way, but if you must buy, the Rugers are the cheapest big brand, amazing customer support and nitride everywhere that matters, machining and fit and finish make the $1100 Colts look like garbage. If you just must spend a little more the Springfield Saints are similar in quality with more bells abd whistles and nickel boron buttery triggers. Just above that, the Springfield Edge are a ridiculous bargain, built like rifles twice their price.
Best of luck.