At this point, there's not much 300aac ammo available and no more brass being sold on GB. What was listed jumped to almost double last week. With this in mind, if you're not a reloader willing to form your own brass, you're not going to have much ammo.
I haven't checked the 6.8 lately but expect the same. One advantage with the 300 is 223 mags work.
If you have a 16" AR I'd suggest getting a 20" and vicie versie.