Copper has a much much better heat transfer coeffidicnt than SS. However, the way it works it that the highest resistance to the heat flow is in the liquid metal interface for both the water and wort side. If you go through the equations you find that the SS is about 87-88% as effective as the copper, even though the heat transfer coeffecient is so much less.
I have copper, as I built my own from a 50 ft 1/2 coil of copper tubing that was about $50 at Lowes, and then add the end fittings an you are over $60. Surface area is a big part of the equation, as that is where the heat flows. If the SS is a 1/2 inch 50 foot design for $40, it would not be a bad way to go..