The reason aluminum radiators dissipate/cool better is because the tubes can be longer. Aluminum is stiffer, and can allow tubes up to 1 1/2" long. Brass/copper is softer, and tubes can only be 5/8" long maximum. Longer tubes, more tube to fin contact, better cooling.
Copper, unlike aluminum, can be easily repaired at home or in a local shop by use of soldering techniques. Aluminum required a process known as tig welding, which may be more specialized.
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