Copper is a better conductor, but the s/s is stronger so the tubing is probably much thinner.
But is the SS used in these Williams chillers actually thinner? I have welded tri clover end caps to these chillers before and don't remember the tubes being particularly thin. Certainly not much thinner (if at all) than the 3/8 copper tubing used in my CFC. If I had to guess, I would say that my copper tube is thinner.