My initial thought on a 7.62x39 AR is outdone by the 300 BLK.
- Ammunition availability: I haven't seen much brass in 7.62x39. With the hoards of .223 brass to convert, this reason alone would be enough for me to lean more towards 300BLK.
- Magazines - Alternate magazine would likely be needed with the 39, whereas it's not with the 300BLK. I say likely because I've never tried, but I doubt it would work as is.
SKS / AK platform for 7.62x39 all the way. I personally do not think the 39 cartridge is THAT good where I'd want to adapt it to an AR platform.