The alloy is selected based on the environment the part will function in the performance requirements of the component.

Magnesium is produced from sea water, brines and magnesium-bearing minerals which offer unlimited reserves.

AA 356 T6 and A356 are very similar in properties with AA A356.0 being a higher purity alloy. In the heat treated T6 condition, AA A356.0 is stronger, more elastic and harder than AA 356.0. 

By far the complexity of the part is the main factor affecting tooling cost.