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next is do you want direct impingement (original design true AR) or piston driven (AK style). these are the ways in which the bolt is moved.
I believe the best AR out there is the one you build yourself that meets your specs, needs, and most important your wallet. Just build one from ground up an you'll be addicted.SS
I have both, the the piston is supposed to be cleaner but I haven't noticed it to be. IMO the piston rod is one more part to have to worry about even though I haven't had any issues with it. The gas blowback uses rings on the bolt and the piston needs a spring behind the bolt. The piston driven usually has some form of adjustment to allow you to adjust the amount of gas to drive the piston.