I see the 300aac having advantages that you can use the stock 556 bolt and magazines, all you need to get it up and running is a barrel. So the up front cost would not be as much if you already own an ar15 in 556 (mrp would take about 1min to do the conversion). Remington and cmmg have ammo available in both super and subsonic rounds. If you do not already own an ar in 556, I agree the 6.8 is a superior round and you will be buying ammo specific magazines, bolts, and a barrel. off the shelf ammo is not cheap for either round so you would most likely reload if you do any volume of shooting.