I had a Saiga 7.62x39mm, but decided to go with AR because of 300 BLK.
The modularity of the AR and the availability of parts due to numerous AR mfg and suppliers.
Except for the stripped lower, which can be easily removed and isolated, all other parts of the AR are not regulated and can be easily bought, sold and traded without regulation (at least in the free states of the U.S.)
AR have feeding issues with 7.62x39mm.
I want to hand load a majority of my ammo and not be dependent on ammo supplies.
Wider range of loading supplies available for 300 BLK.
300 BLK use commonly available .308 bullets. (7.62x39mm uses less common .311 bullets.)
Supplies of brass for 300 BLK are virtually endless (made from .223 and 5.56 brass).
Supplies of 7.62x39mm brass are almost non-existent.
Imported (i.e. cheap) 7.62x39mm could get regulated out of existence.