Inf-Art and Arm-SP-Art are the most effective units.
Mtn-Art and Mech-SpArt are more tc-efficient. Mech may suffer higher manpower losses than Armour, but repairing Armour takes 50% more time while using Mech instead of Armour allows to use one third more mechs thn armour per tc available. Thus the losses in terms of strenght may be lower.