You'll get opinions from all over, but for my money there is precious little to be gained in an AR with a piston upper. Longevity and durabiity? Quality DI uppers will go 30,000 rounds or more before major parts will need to be retired. Everybody and his brother-in-law has spare AR parts if you need 'em, whereas nearly every piston gun has proprietary parts - give that some consideration.
My ARs are an 'original' (2004) American Spirit Arms which has about 10K rounds through it without a burp, and an almost-new CMMG. Both are mid-length gas systems with 16" barrels for greatest flexibility. If I bought today, I'd probably look closely at a Bravo Company, LMT or Colt, and go with direct impingement and not look back. I don't think you can get just uppers from Colt, though.
When you get your gun together and sorted out and have a couple thousand carefree rounds thru the gun, check out Pat Rogers' EAG Tactical for some serious training. He runs courses in Casa Grande typically twice a year. You'll learn more than you can possibly retain and gain a huge amount of confidence in your ability to fight with the carbine - if that's important to you.