Kind of off-topic, but for England, in custom battle they have Armored Sergeants, but they're nowhere to be found in the campaign. And I don't think they're supposed to either because it's not in the unit recruitment list poster thingy that came with the game. Weird why they're included in the custom battle roster.

But anyhoo, Levy Spearmen are exactly the same as Militia except for the upkeep as econ mentioned. Kind of weird stuff with some areas of the game, but of course, the great modding communities will fix all that.