Cast iron heads are more resistant to warpage than aluminum if the engine overheats. Since cast iron retains heat more than aluminum, equal performance can be had at a slightly lower compression ratio. Coolant passages are more resistant to corrosion/erosion with cast iron heads over aluminum due to electrolosis from a lack of cooling system maintainance. Performance cast iron heads cost less than aluminum. The cast iron valve guides are more resistant to wear than the bronze alloy guides found in aluminum heads. Spark plug threads don't strip out as easily with the cast iron heads as well as the threaded accessory mounting points. Aluminum heads expand at a greater rate than cast iron which means that head gaskets are subjected to a greater amount of surface shifting when using a cast iron block. The head gasket's cyl. sealing rings can etch grooves into the aluminum head's deck surface due to the shifting. Aluminum heads need to be retorqued more often due to the greater expansion/ contraction rate. Aluminum heads can shave 50 lbs or more from total engine weight. Aluminum heads are easier to machine and port than cast iron. Aluminum heads can be welded or repaired more easily than cast iron.