Aluminum conducts heat twice as fast but needs to be twice as thick so it is a wash. The real issue for functionality between the two is the vast difference in mechanical desighn NOT the material. The OEMs do it in AL because it is cheaper, they also use those stupid plastic tanks on the end because they are cheaper but usually around 5-10 years old they start leaking and need replaced, especially with dex-cool type crap coolant. Many brass radiators (includeing brass tanks) are still in service 50+ years later .