Aluminium V Iron Heads. Aluminium transfers alot more heat out of the combustion into the water jacket than does iron. This reduces the efficency of the motor & means you then have to have a larger radiator to get rid of the extra wasted horspower. Because the combustion chamber is now cooler you can increase the compression ratio to compensate for the lost HP but you are just making up for the reduced efficeny. Most people would say it's obvious Aluminium is better because you can run higher compression ratios, but thats pretty simpilistic thinking.