The gas piston systems are supposed to be more reliable, which ARs come with this feature? Are there any ARs that come with a crisp adjustable factory trigger like the ones on Sako/Tikka bolt rifles? All the factory triggers I have seen thus far have been very heavy (7-8lbs) and no mention of a user adjustable feature.
Gas piston is a more reliable design, but more importantly, it runs cooler and isnt spewing hot gaseous carbon fouling into the receiver in order to run the action, therefore also running cleaner. Downside is that gas piston, while not a huge deal breaker, is a bit heavier and costs more. Imagine an AR with AK-like self loading system. That's almost too sexy.
You'll hear that DI is as reliable as GP. It's also easier to clean a GP that it is every nook and cranny of a receiver due to the goofy design of DI. The weight difference is minute.
Degrade accuracy? Only if you suck.
Who makes em...
Ruger 556
Colt 6940P
Sig 516
Smith and Wesson M&P psx...to name ONLY a few.
Also, there are several conversion units that allow you to change from DI to GP if you decide to do so later.
As much as I love GP (I'm a diehard AK fan) I'd get the Sport. Spend $700 overall and use the rest of your money for ammo and gadgets, an AWESOME cleaning kit as you'll need it with the Sport, or a whole other gun. But, as you know having had an M16 back in the day, do not be fooled. DI will work, but it is a dirty system and favors the cost inhibited who LOVE to clean their rifle. My Sport shoots great, is accurate, and is light. The Smith PSX is only a 1/4lb more in weight, it's GP, and just as accurate with 64 gr pills. It also doesn't such to clean. I'll talk my brother in law out of it someday
I won't do the DI vs GP arguing if it gets sparked. They have their ups and downs. GP has more ups for me, YMMV.