Not suggesting a plastic cap vs stainless, only including them in what's available. So go with what you want to pay for. If it were mine, I'd prob'ly choose plastic for a couple of reasons: 1. there's no chain--at least there shouldn't be--on a waste cap, so unless you're extra careful to stow it where it can't fall overboard
, sooner or later it will and plastic costs less to replace. 2. If all the other deckfills on your boat
are stainless, a black plastic cap makes it a lot harder to pump
out your fuel
tank. But to repeat...I'm not recommending either, just showing what you can find for a reasonable price
if you use the item you need as your search term instead of only looking for it at the usual marine
equivilent of "big box" retailers. You'll be amazed by the number of retailers who have what you need for a lot less.