If you use your cooker often and season it properly before you use the grates, I don't really see any need to spend the extra money on the SS. In the cookers we build, we only use mild steel. If it is seasoned by spraying down with oil and baking it in at a higher temp for a couple of hours then you shouldn't have problems with rusting. If your cookers sits for long periods between cooks then you may need to consider the SS.
For the charcoal grate I think SS will stand up to the heat a bit better and fend off the rust for sure.
Just my $.02. Good luck with whatever you choose.