The main difference between the two is pricing and the outer shell of the mufflers. Stainless Steel cost more than Aluminized Steel. The Stainless steel mufflers have a polished stainless steel shell and are 100% stainless construction. Nothing looks better or lasts longer than stainless steel. The aluminized mufflers have stainless internals and necks but have an aluminized outer shell. Also, the aluminized steel has only a protective coating that, if scratched off, may rust. Both mufflers have identical internal construction and materials and both offer you a lifetime warranty and will give you that Straight-through Wide Open Performance with no restrictions. No restrictions equals better flow, and better flow equals more horsepower. 

Additional Info Regarding Stainless Steel and Aluminized Steel Body Mufflers:

• MagnaFlow Technology
• Universal-Fit Mufflers