When the North Koreans, Chinese, and/or Russians invade - you'll want the 7.62x39.
-J-
Quite possibly more than one!When they do, you'll be able to just pick one up off the ground.
When they do, you'll be able to just pick one up off the ground.
When they do, you'll be able to just pick one up off the ground.
The question cannot be answered simply.
It depends on what you want. The 300BLK can be used readily and reliably in the AR platform with the ONLY change needed being the barrel. The 7.62x39 can't be easily used in the AR platform (there are uppers available but most have issues). If you like AK style rifles then you want the 7.62x39. If you like AR style rifles then you are better served by the 300BLK.
300BLK ammo is less available commercially and is more expensive than 7.62x39, but it is easily made from .223/5.56 brass and uses a wide range of .30 cal bullets if you reload.
If you want to suppress or go SBR or both, then the 300BLK wins hands down as that is what it was designed for.
When the North Koreans, Chinese, and/or Russians invade - you'll want the 7.62x39.
-J-
And only the North Koreans still issue large amounts of 7.62 AKs, the rest have gone to newer chamberings. And there is exactly around a 1% chance either of the 2 Asian nations elluded to will be able to deploy troops across the worlds largest ocean and reach our shores our life times. For us it makes more since to get stuff in chamberings our alphabet agencies use.