There's really so little difference between most AR's on the market, flip a coin. Finish will vary according to price, so really all you need to focus on is the barrel and the trigger. Everything else is personal preference for looks, component features, and physical fit. If you look into customs or semi-customs, you generally get what you pay for, but a lot of what you're "getting" and "paying for" is better looks. The reality I've seen in AR's in the last 10-15yrs is that there's a lot of distinction in the market without much real difference.
I've had Colt, Armalite, Bushmaster, DPMS, RRA, Oly, and one Black Rain (the skulls sealed the deal, grew tired of it after 3mos though). I've also built over 3 dozen of them from component parts, almost exclusively on Bushy uppers and lowers (a few RRA's in there).
Piston drive is something worth paying for, although it's hard to convince yourself that you should pay for it. It's highly over-rated, but if there's a "third thing" - again, the first two being trigger and barrel - that I'd add to an AR, it's a piston drive.
*** This is all said with the premise that you're talking about free-float models only. I don't care for delta ring rifles, no reason NOT to float them. ***