I've used the spring loaded arms and the ridgid ones and what I have found is that the ridgid arms "force" you to move and angle and use footwork more then the spring ones.
Since the springs have give, we end up moving the arm into position instead of moving ourselves.
I think that a ridgid spring mounted head would be cool or a hard "anatomically correct" body would be better.
I am not a fan of putting it on wheels/casters, but I do like it when it "moves" with my strikes and it should be well weighted.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !