You'll be fine with stainless. Why?
Well every plug I've ever used has been brass attached with stainless fasteners and I never had a problem so it must be right.
The real truth is that brass and stainless are fairly close on the galvanic metal scale and thus have a lower tendency to create the reaction that would cause one to erode faster than the other due to galvanic corrosion.
You choose which answer works best for you.
The real key here is the condition of the media surrounding the hole. That's the most common place for failure.
Good luck.
EDIT: I see SB beat me to it.
