Most of the bigger aftermarket companies, Dart, Merlin, etc. make good heads in both material. I think the aluminum will save you a good deal of weight and are a little less sensitive to detonation due to the fact that they don't retain as much heat in the head. They are also easier to repair in case of the dreaded valve failure. And lastly, they look so cool when you get the outside of them polished prior to installation!
Iron will save you money.
My 2 cents, and it might not even be worth that much!
Craig