Bro 300 AAC is about the single most difficult round to find right now as most manufacturers have switched it out for 556. That said, yes it’s 100% preferable to x39 as a platform. Totally.
AR-10 vs 15 is just a totally different animal. Ballistically, it completely depends on what you want to do w the gun and what you’re referring to (velocity, energy, range, weight, etc etc). Our specific context was about Ammo availability/cost. 7.62x39 is going to be far and away the safest bet, as of today, in that realm.
Again, just all depends on what you’re trying to accomplish. If you want a longer range weapon, sure go w the AR-10, go with 6.5 Creedmoor.