The performance advantages of 300 blk over 5.56/.223 are incremental and dubious.
The lighter ammo results in greater combat load which means better persistance and better chance of gaining fire superiority.
Also its MUCH cheaper (see my thread in AR forum) and so can be stacked deep .
The latter is incredible importance as we dont know what our ammo supply situation will be long term
Not an expert here. I just look at what times we may need our firearms the most. To me it would be during a SHTF situation. If for some reason I stayed alive and for some reason had run out of ammo for my firearm, what would be the easiest cartridge to find while scavenging. .223/5.56 Should be plentiful. Everybody has an AR of Ruger Mini 14. Many houses also have a .22. So I'd want to stick with the common calibers. Not sure 300blk would be considered a common cartridge.
And the trick is to have something that will fire that caliber. A nice little SKS does that nicely. Or an AK if that's your cup-o-tea. I think you can even build an AR in that flavor. It might be as simple as a barrel swap to change calibers on some weapons.
I would stick with the 5.56/.223 myself. I have found that everywhere I look you can find that ammo and the price is not to terrible. . . . The more specialized the round, the harder it is to find and the more painful it is to use. Think of Grandpa's Rifle/Pistol set he has. Finding ammo for it is a real pain in the ass because its such an older style weapon and has much more specialized rounds.