Aluminized steel is an invention of recent years for manufacturers to save some bucks. The process involves taking standard-grade (lower-grade) steel and coating it in aluminum. It corrodes faster, and is of lower quality. It also rusts from the inside-out, so it's tough to find a rusting spot until it's too late.
t304 and t409 are both stainless steels, but t409 is more susceptible to corrosion of the two (thus why t304 is the optimal choice).
There is no significant performance advantage to either t304 or t409, it's more about longevity and durability of the products you buy.
For us, it's t304 or bust!