There are advantages and disadvantages to both materials.

Cast iron cylinder heads offer the benefit of being inexpensive, durable, required in many oval track classes, and a great performer in many street performance and drag racing applications. Cast iron heads are also heavy, less forgiving of high compression ratios, and more expensive to port.

Aluminum cylinder heads have the advantage of being lightweight (generally 50% or less of the weight of a comparable iron head), repairable, easier (which translates to “cheaper”) to port, and more forgiving of high compression ratios. Disadvantages to aluminum heads include cost (compared to iron heads), the need to pay close attention to proper fastener tightening procedures to avoid thread damage, and issues of valve lash and other adjustments that may be effected by the fact that aluminum expands quite a bit more than iron when your engine reaches its normal operating temperature.

Which is the best to use depends largely on your application, and which properties of the two materials best suit your needs. In general, most street applications can use either material equally well –keeping in mind that it is best not to exceed a compression ratio of 9.5:1 when using cast iron heads (10.5 is a good upper-limit ballpark with aluminum) in a street application. Most race applications will benefit from the weight savings and ease of porting afforded by aluminum cylinder heads –unless of course, you are running in an “iron only” racing class, which will mandate the use of cast iron heads.