Aluminium has near as dammit the same heat conductive properties as copper, esp as most standard rads dont contain much copper anyway! Look how many things in life use aluminium as a heat sink, PCs, Lights, TVs etc etc.
Aluminium rads also usually have alloy end tanks rather than plastic... Also the fins on an ally rad, are alloy and welded to the core so no loss of heat transfer, whereas copper rads have steel or copper fins that are soldered onto the core, solder doesnt conduct heat very well at all...