"300 blk vs 7.62x39
Which is better and why?"
I don't have any use for the AKs or their ammo. This doesn't mean that it is bad/no good/(insert your own adjective).
AKs are loose. Run dirty and just keep on going.
ARs are tight. Some run dirty, most like to be clean and well oiled.
AKs have 'crap' ammo. It is cheap and plentiful. Not great accuracy, can do better (some say much better) with good stuff.
ARs have good and consistent ammo. Quite accurate for most.
AKs, military surplus is 139 or so grain FMC. Not the best bet for hunting.
ARs, military surplus is 55/62 grain FMC/steel core. Not the best bet for hunting.
But we are talking about the 300 AAC Blackout.
An AR built for the Blackout is tight and accurate. Great ammo with bullet weights and types for all needs from 110gr to 220gr and more.
No, it is not fair to compare great loaded stuff for the 300 BlackOut to crap stuff for the AK.
The AR in 300 AAC Blackout is the standard AR15/M16/M4 configuration with a carbine gas system and buffer. Same everything except barrel and tend to favor shorter barrels at that.
But which is better? Personnel preference. If all you want to do is bang away, go with the AK. The 300 BlackOut is more expensive, but then they are no more expensive than any comparable AR in .2233/5.56. Factory ammo (I have never shot any) looks to be reasonably prices. I cut down dead .223/5.56 brass, form it and trim it, same primers and have my selection of bullets. Oh, it uses less powder than it's little daddy round.
So, pick your weapon. I've picked mine..... But I still have and like my other ARs.