I manufacture aluminum radiators for vintage cars, so allow me to give you the inside scoop.
First if all, your first consideration is aesthetics vs practicality. With only a handful of exceptions, aftermarket aluminum radiators won’t have anything like the look of the original. If your goal is preservation of a classic, you really should recore your stock radiator. Nothing looks more ridiculous to me than an aluminum radiator in an otherwise original and well preserved old car, and I make these things.
OTH, if you are looking for a practical radiator for a driven car, copper is expensive, and more importantly, becoming very hard to find. I can’t think of a single old fashioned rad shop still operating in the Hudson valley. So you will be at the mercy of specialists and a diminishing old stock of parts. Not an unfamiliar situation for the restorer, but not the best for the practical driver. A custom made aluminum radiator will last 20 years or more, but will not be repairable or rebuildable.
As for why aluminum works better, it’s not the metal that matters. The secret is fin-tube contact. This is optimized when you have a small number of large tubes. So going from a radiator with three rows of 1/2 tubes to a radiator with a single 1.5 inch tube is actually going to improve heat transfer 15%, regardless of material. Most of my cores have just two rows of 1” tubes. When you enlarge the tube, the walls have to be thicker, and the advantage goes to aluminum because of weight. The rest of it is that there has been a lot of development of fin materials, and modern corrugated fins offer more surface and create more air turbulence than old fashioned accordion pleats. That’s the engineering part, everything else is rumor. Finally a new radiator is cleaner, no matter what it’s made of, and that has to help.
Because there are is so much ‘common knowledge’ from the days of copper, there are a lot of bad products out there. Beware of Chinese radiators that advertise four row aluminum cores. These abound, but they are not worth the price, however cheap. Again, fewer, larger tubes is better than more small tubes. And higher fin count also means less air flow, so again, what you remember about copper doesn’t help.
Finally, corrosion. Here’s the secret: 50 /50 coolant. Don’t listen to the water cools best crowd, and never mind that you never drive in winter. Antifreeze does more than protect against freezing. I recommend a low silicate antifreeze, like Zerex G05, rather than a dexcool. But whatever you choose, stick to a 50/50 ratio.