Back in the day Stoner was a good brand. It was ran by E.M. Stoner, they man who built and designed the first AR rifles. I couldn't tell you about AR Stoner today, though...
If it's a good price, and you like the upper, give it a try and post up your results that way others will know it's still quality stuff. I'd be interested in reading your write-up about it.
I personally prefer the .300 BLK to the east-block 7.62x39... The .300 BLK seems to be a more accurate cartridge, too. Most AK's in 7.62x39's are not very accurate, from my experiences with them. 100 yards, you can hit someone in the groceries consistently, but not much beyond that as far as accuracy goes. Now, that said, I have not shot an AR-platform in 7.62x39...So I can't comment on that.
I do have a custom .300 BLK upper that I built back in 2015...Awesome cartridge. It keeps defying the critics and keeps amazing the fans. If you break it down for what it is, it is the most efficient 400 yards cartridge available today. Even my supersonic loads with 168 Bergers hold less that 15gr of H110... The most expensive part of reloading .300 BLK is the bullets. Surplus 5.56 brass I pickup at the range, then process them myself, and less than 15gr of powder, and a $0.05 primer, and you can reload super-high quality .300 BLK for around $0.50-$0.65 per round.
I went with a 16" barrel, so I wouldn't have to SBR my lower at the time. Now that I have the extra money, I'm thinking about SBR'ing my lower so I can drop down to a 10.5-12.5" barrel, and then put a can on it. I'm also waiting on the Hearing Protection Act to pass before I go buy a can... I hate waiting, which is why I haven't gotten one yet.