The Adams Arms piston gun is a solid value at $599. They sell just the upper for $575, so its like getting the lower for $25. You do have to factor in an optic or sights to that price. Sights will set you back at least $100 and up to $200, depending on what you buy. If you go the optic route, the cheapest quality optic that wont fall apart on you is a Primary Arms or Vortex red dot. Those will set you back $100-$150.
The Smith & Wesson sport comes with sights is ready to shoot out of the box, and is a solid value.
My best advice? Buy the Colt 6920 for $899 on sale right now from Palmetto State Armory. Buy once, cry once is my mantra. It'll do everything the others will, but has a higher resale value and better quality components. Plus it makes a great SHTF gun if law and order breaks down. Top it off with an Aimpoint Pro red dot when funds become available ($360-$400) and you will have a solid, dependable weapon that you can be proud of.
As far as piston vs direct gas impingement (DI), the DI is lighter, has less parts to break/replace, works perfectly fine and is the standard which makes getting parts much easier. Piston vents the gas at the gas block instead of the action, so they generally stay very cool and lubed longer. It's not really a pro or a con in a sense since DI are just as reliable. Just realize that AR's like to run really wet and work better when run really wet and re-lubricated often.
Palmetto State Armory runs EXCELLENT sales often. You can get their complete $150 lower and one of their premium uppers (around $399 to $499) and have a mil-spec quality weapon with a high grade FN or FN hammer forged barrel. That would be a good choice too. You have to check their website often as some items go in stock and out of stock several times a day.
PSA lower $149
http://palmettostatearmory.com/index.php...o-magazine.html
PSA premium complete upper $419
http://palmettostatearmory.com/index.php...ing-handle.html