Aluminum radiators are lighter. They can cool better, but so could a brass/bronze. It really depends on flow rate, and surface area. Since the metal is relatively thin, the heat transfer capabilities are all fairly close. A stock radiator will cool a stock motor just fine.... They are built for that...

I take my above stock motor, go offroading in the mountains in 90+ degree whether WITHOUT a fan shroud pulling 3K RPM's in 1st gear low-range up steep hills and don't manage to over heat my jeep...