Copper or Cast Iron for Frying
Is Cast Iron better for frying breaded Fish and Chicken? Or am I better off using Copper with the good heat response?
Cast iron. Copper has good response, yes, but what you really want is good heat retention. When you drop the chicken/fish in the oil, the oil drops in temp rather rapidly. You want to keep the temperature from going too low too fast and the cast iron will help with that. Cast iron tends to be much heavier and when dealing with gurgling hot oil, a solid, sturdy, heavy pan is a good thing.
Very loaded question! Without a doubt the cast iron will do a fine job. Copper can do as well, maybe better, but that depends on the thickness of the copper.
A good copper skillet or saute pan (2.5 mm) will actually weigh about as much as it's cast iron counterpart. A pan like that will be just fine. Lighter stuff (1.6mm or 2mm) might not work as well.
Of course, you could buy one of each size and type of skillets that Lodge offers for the price of one copper skillet. Copper definitely has its fans that claim the cost is worth it.
Both cast iron and copper would be fine for pan frying where you want even heating, but for deep frying fast heating of a much large volume of oil is important, and that's where copper has the advantage. As for the finished product, either will do the job. Copper will just save you a few pennies in energy costs (which means a $120 copper saute pan will pay for itself in about 200 years compared to a $20 cast iron pan). Ok, I pulled those numbers out of thin air, but you get the picture.
Heat response is the last thing you want. But if you're using thick copper (which has less heat response than thin copper), either will work fine.
The biggest problem when you're frying is temperature drop. Put cold food in hot oil and you have warm oil - which guarantees greasy results. A cast iron (or heavy copper) pan is useful because its thermal mass minimizes the drop in temperature.
Of course, a burner with sufficient BTUs can address this problem, too. If you have serious (eg, commercial / outdoor) firepower, I'd say that copper might be superior because it will spread the heat more evenly. But you're not going to go wrong with either.
I can attest to that after my wife brought home some very thin porkchops. I breaded them and fried them in cast iron in very hot vegetable oil with pretty good results, sort of like schnitzel. As Alanbarnes says, cast iron retains heat very well, and the coating ended up being quite crispy since the heat doesn't drop off much when you add the meat.