409 Stainless is generally considered a lower grade of stainless, but with very high durability and toughness. For this reason it is widely used in OEM exhaust applications. Although it can be polished, it is subject to "coloring" when exposed to high temperatures. 304 Stainless has a higher nickel content which allows it to accept and maintain a mirror like finish when polished. As always beauty has a price, and the higher nickel content makes the material more brittle and susceptible to cracking.