If I have to choose one or the other, then it's Masterwork Chainmail. If I can choose any armor, then it's probably leather, unless the computer is bringing enough ranged firepower that I can't make theirs trivial in the opening round or two of the battle with a big alpha strike. Regardless, I'd never use plate on anything except a tank unit, because that -6 initiative is not worth working around, especially on a ranged unit which already has at least -4 initiative from the weapon. -4 initiative from the weapon and -6 initiative from the armor might be okay if ranged weapons could be heavy hitters like the clubs are, but they don't have that kind of striking power and so you'd be better served by not crippling their initiative.

Getting the first strike is a rather big deal in this game. Stacking initiative penalties on an already low damage unit like an archer just isn't a good idea unless you really, really need the durability.