Wort chillers operate by transferring heat across a surface - the hot wort and the cold water set up a temperature gradient, so heat flows between them across the material of the chiller.
Copper has a thermal conductivity of around 380, while stainless steel has a conductivity of around 20. If you built two chillers that were identical apart from the material, then the copper one in theory could have a heat transfer rate 19 times greater than the steel one, chilling the wort 19 times quicker.
But that's just theory. In practice other factors come in to play. the most significant being that it's hard to maintain the temperature gradient - wort that's been cooled and water that's been heated tend to stay in contact with the chiller surface, which reduces the temperature gradient and the efficiency of the chiller. Some measures can be taken to help prevent this, such as ensuring the wort and water are in constant turbulent motion by storring the wort or using a convoluted coil.
But by far the biggest factor affecting performance is chiller design. Most stainless chillers are plate chillers, while copper is used typically in immersion or couterflow chillers. Here the thickness and total area of the material are a key factor in efficiency, as well as the material's conductivity.
A typical plate chiller has a surface area of 0.7 sq.m and a thickness of 0.3mm. A cfc has a surface area of 12' of 0.5'' tube - 0.14 sq.m. And the thickness is 0.028'' or 0.7mm. The figures show that although the stainless has a lower thermal conductivity, there is much more surface area than the copper. Plugging in all these figures, that makes the copper about 1.6x as efficient as the steel.
This is only a back of the envelope calculation. Actual efficiency is still dependent upon the specific design of each chiller. For example, although some CFCs do use convoluted copper to create turbulent flow, it may be that the relative size of the turbulence is much smaller than compared to the ridges in the plate chiller, making the plate chiller overall more efficient.