You are asking to compare apples and turkeys. Two different calibers, two different cases, two different chambers.
Neck length isn't a huge issue in accuracy in my experience, as long as there is enough of it to hold the bullet. I mean is there really a significant accuracy difference between the 243 Win and the 6mm Rem?
Before I pontificate on which cartridge should get a premium seating die, why don't you experiment with your rifles, and see if you can figure out which chamber is more tolerant of runout?
Machine guns are awesome until you have to carry one.