I'm a hydraulics design engineer, part of my job is calculating delta "T" across heat exchangers. There are so many factors to include in a question like this, that it becomes a giant project! but, bottom line...if the water flow, inlet temp, and air flow are the same through both radiators, the aluminum will reject a minimum of 35% more BTU's. Aluminum is a better conductor of heat than copper. One other critical factor, ONLY use enough anti-freeze to protect the engine from freezing, the higher the consentration of anti-freeze the less BTU rejection you will have! Good luck.