The biggest differences between copper/brass and aluminum rads are the tube widths inside. Most aluminum rads start at 2 rows of 1 inch tube width and go up in tube width from there. That width is more flow which is what helps keep temps in check. Width also equates to a wider tube surrounded by fins which in turn has more surface area for air to flow over and remove heat.
Copper rads are usually rows of tubes that are 3/8" wide which means 2 rows of copper are actually less in flow capability than 1 row of aluminum 1 inch tube.
Now I have nothing against copper/brass rads as they have been around forever, but alot of times guys will use a rad they salvaged from another car cause it fits. One must remember for example....a rad from a 69 Camaro w/a 327 /250 HP motor will not cool a 572/800HP blown motor.