Very interesting question and I know the answer!
I'm reading Tony Boyer's "The Book of Rifle Accuracy" and he states that lightweight firing pin are superior for position shooting (standing, kneeling and prone) but normal/heavy firing pin deliver superior accuracy because of more consistent primer ignition.
For prone shooting, it seems that normal/heavy firing pin is the way to go but if you're shooting NRA High Power rifle, you're better off with a stiff spring/light firing pin.
Alex