Salt spray comparison with 409 SS to CR steel

We are a manufacture of portable magnetic signs. The sign panels to which the magnetic letters are applied is currently made of CR steel that has been e-coated as a primer and then top coated with a white powder coat. We have been exploring the possibilities of using a 409 stainless since it still has magnetic qualities. My question is will the 409 stainless with one coat of powder provide a better salt spray test result than the CR steel with the e-coat primer and powder top coat? Also the panels are 3' x 6' and are perforated with 1/4" dia. holes on 5/8" centers - which produces alot of edges.

409SS should give considerably better corrosion resistance (but not great compared to 304) than CRS, but is not as magnetic and costs more. Before I jumped into this in big way, I would try a few to see if it was worth the extra cost and was magnetic enough.