Larry, you are right. I've seen a number of post stating that aluminum is thermally "better" than copper. As far as thermal conductivity is concerned, copper is better. As far as heat capacity (in layman's terms, the amount of heat the copper, or aluminum, itself, can absorb, per unit volume), aluminum is better. In terms of thermal properties per value of material, aluminum is better--in other words, you can build a better heat sink, with aluminum, for ech dollar spent.

But given the same size heat exchanger, copper has better thermal properties.

But there are many other factors. Copper, generally, costs more to fabricate than does aluminum. Copper has greater corrosion issues, with respect to A/C systems.