I built a 7.62x39 AR for my nephew as a wedding gift. I loaded him up with the Duramags he says they work great.You can have AK47 power in an AR15 platform if you buy a 7.62x39 upper too...
IMO the only advantage to 300 bo is the ability to run sub sonic. Given the cost of 300 vs 7.62 I elected to go with 7.62. I also had several issues finding the right combination of ammo to run the 300bo upper I had, so I sold it for a nicer grendel SBR upper.