I am also new to the AR family. Did the same research as you are doing, and looked at:
Stag / Windham / S&W SIG M400 and a few others. Ended up with a Core15 based on some positive reviews and comments from my LGS.
The Core15 was also built here in FL and I wanted to support a local business.
Availability was also a concern, at the time some of the more expensive AR's had long lead times.... Bravo Company, Spikes... others were out of what I wanted to spend.
Anyway..... I'm at almost 1000 rounds, and have had only one failure with brass ammo. (Fail to eject/double feed caused by ME not seating the mag properly). My particular AR doesn't like the Herters steel cased ammo. It appears from reading a LOT of threads.... steel cased ammo in AR's is on a case-by-case basis... some have no problems, others can't make it fire.
I am very happy with quality, fit & finish and accuracy. (Still need to work on my part to make it shoot better) It also has a lifetime warranty. When shooting it side-by-side with a Colt, Bushmaster (early) and a S&W, I was very happy with quality that was at least as good as any of those.
Would recommend maybe taking a look at the GTO Core15 if you haven't already made a choice.