"Aluminized steel headers are the cheapest option for a header material, but even in a dry, warm climate with little precipitation, they will eventually corrode, which is why they are usually ceramic coated. The ceramic coating offers better thermal resistance and also helps retaun heat in the header pipes which helps velocity and flow. Stainless steel is resistant to corrosion. It is also non-magnetic and compared to mild steel, has superior high temperature properties, especially practical for exhaust systems or a header where high heat is typically found. Stainless steel has half of the thermal conductivity of mild steel and retains heat in the headers pipes much better. This improves velocity and scavenging, a reason why stainless headers make more power."
-Sarah Frost