An AK is a fun gun (says the guy who owns an SKS because there are such restrictive laws in CA re: AKs and ARs). If you want a fun gun get one of those two.
I can’t see a reason to get the 39 in any other platform than one of those two. At all.
Let me start out by saying I am not a huge fan of the AR platform. I own them, but when I want to shoot for fun, It is usually a milsurp or something like it. Whether it is a WWII bolt action, Garand, M14, or Mini 14, these are what I generally shoot for fun. I am an unrepentant iron sight shooter and am pretty good at it. One of my never-going-to-get-rid-of-guns is a Chicom SKS that I shot a 95/100 out of our last range milsurp match (losing to another Chicom SKS...if I knew he had replaced the sights I would have pulled out the M14 instead.) It was a wedding gift from my father in law. Good start to a marriage
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And I still say, of your choices, get the AR. The SKS and the AR have both been 100% reliable with steel ammo, which is the major pull with 7.62x39. My SKS has outshot any AK I have put my hands on. The AR is more accurate than the SKS. Every time. And that is the cheapest 16" upper that PSA sells in 7.62x39.
If you are using Irons, the sight radius on my AR is longer and the carry handle aperture sight is MUCH better than the blade sight on the SKS. I did grind the sight off the FSB and add a cheap rail to the AR to push out the front sight.
If you want any kind of optics, the AR wins against just about anything.
With that said, I understand the desire for something new and different. If you want an SKS or AK, don't let logic stand in your way. I did mention I am more likely to shoot a Mini 14 than an AR-15, right? I end up shooting 3 rounds in my 7.62 AR for every 1 in a 5.56 AR. The only AR that gets shot more is the 9mm.
Even though I reload, I don't reload 7.62x39 in large quantities. And 7.62x39 is tapered, so steel ammo extracts from it better than steel 5.56 ever will. So if I want to shoot, and don't want to reload the empties, I shoot the commie AR. 18 cents a round for 2-3 MOA is not bad.
If you are looking for something completely different, look at the Ruger American Ranch in 7.62x39. The bolt operates roughly (like a Mosin) but I have gotten some sub-MOA groups of 5 from Wolf Polyformance Ammo. Yep, the cheap stuff. The normal group with this ammo is 1.5-2 MOA. I dare you to find a bad review here for that Rifle. It is an unbelievably handy bolt action. If they put 1903A3-style sights on it with a tapered barrel, I would hit the range with it just about every time.