When choosing between 304 and 409 stainless steel for automotive exhaust systems, it is essential to think about corrosion resistance, temperature resistance, cost, and cosmetic preferences. While 304 stainless steel offers better corrosion resistance and looks, it comes at a higher cost. On the other hand, 409 stainless steel provides good corrosion resistance at a more affordable price point, making it a practical choice for many automotive applications. Ultimately, the selection depends on the specific requirements and budget of the project.