What you seem to be discounting is the fact the panther have a considerably longer reload/cool down. The comet on the other hand he much better rear armour, speed and accuracy on the move. Once the panther is forced to move any advantage it had was gone. Missing a shot for a panther is not a good situation. As for the veterancy, comets get much faster and gets better turret rotation. Generally speed/mobility/consistency > amour/health.
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