In a SHTF situation, .223/5.56 will be a whole lot easier to find (think scavenging) than the 300blk. That's just my opinion. Your mileage may vary.
Another option is an AR10 (.308. Uses the same bullet as the 300blk). I've heard you can also use 7.62x51 in a .308 gun. So again, while scavenging, you have two option for ammo in the AR10.