Copper is a much better conductor of heat. The thermal circuit is the condition from wort to chiller, chiller, chiller to water. The transfer to the liquids controls the equivalent resistance. In other words, those are the big resistors, to the metal is a small resistor, so making that bigger or smaller has only a little change in performance.
I knew copper was better. All I can say is between the JaDeD Hydra and the Blichmann Hellfire, my brewdays are much shorter.