im guessing that will proably be worked out in the final stages of the game when they are working out balances. As you dont want to have a instance where a few battalions of one unit becomes unbeatable by any other. (like an army of hv kataphracts (or however you spell it) in TW.
therefore i would imagine you would have diffrent units. The heavy armoured attackerr units (like trolls) or the armoured defence units (like the human warriors). And thoose inbetween. they would most likely i would imagine have diffrent styles of fighting with regard to agression and block/attack ratio.