7.62x39 has gone up in price quite a bit. Cheapest is 37 cents a round on Ammoseek, vs 40 cents a round for the cheapest 300 BLK.300BO is basically 762x39 but more expensive and less available.
I have both 7.62x39 and 300 BLK, but thinking about getting rid of my 1 AK and selling the ammo to just stick with 300 BLK. Really don't need 300 BLK for anything, but if I can get .30 caliber AK performance by swapping an upper on my AR, why not? I enjoy shooting 300 BLK more than my 5.56 ARs, it's not as blasty. Especially with a 10.5" upper. But if I had to get rid of 300 BLK, 5.56 does everything that I might need to do with a carbine.