I guess this is as much a history question as a gameplay thing, but whatever.
Did they have markedly different compositions? Were they used differently in battle? Is Armored Cavalry considered a hard target for Comb Arms purposes? Overall, it seems like Mech costs a little less and has much better stats, except that AC has +3 HA and +2 Suppression.
Is there anything else significant about them? Does anyone prefer one over the other, or do you find a use for both?