Aluminum does not retain heat better, sorry suicide bomb. You will typically find better ports in aluminum as cast because it is easier to work with alum. from the foundry so there will be more choices. If all things are equal you will usually make more power with iron, but the weight difference will more than offset this on the track and the ease of repair. Cost is one thing to look at though.
All things being equal: flow, valves, chambers etc. You can build more power with the aluminum because of the ability of aluminum to dissipate heat more readily. You can usually get at least 2 to 3 degrees more advance without the fear of detonation. There is also the bonus of less weight which means faster times at the track.