304 is the high grade surgical stainless steel that can be left unpainted and will not corrode or rust.
409 is the lower quality ss that needs to be painted. IMO stay with the 304, or with painted mild steel. No reason to pay for 409 ss and still have a corrosion issue.
Just don't go with the little Western Stainless Steel spreader. A guy I work with has one, and he really does not like it. He said he spends more time unjamming the salt than he does spreading. Just FYI