I think you'll have limited responses to "Why a 30 degree shoulder (Mashburn) vs a 40 degree shoulder (Ackley)."
There might be a difference between the two, but they would both fire factory 25-06 ammo and then you'd have fire formed brass. I doubt you would notice an accuracy difference, or a velocity difference.
Not trying to dissuade you from your search, though. I vote Mashburn. Because it's unique, and once you have the dies and brass, there's really no difference in shootability between the two.
