300blk does better with short barrels. The main advantage of 5.56 is price and availability of ammo. I'd probably go with 5.56 if the barrel is 10.5" or more. 12.5" 5.56 uppers are highly underrated. I personally went with a 14.5" pin and welded, that way I didn't need a pistol lower.

The main reason I don't own a 300blk is I worry about getting the ammo mixed up with some 5.56. I really don't want to chamber the wrong round, I believe that ends poorly either way. I pull out the Tavor 7 when 5.56 isn't enough.