a few facts about stainless:
stainless uses other metals, nickle etc to "keep " it from rusting
it's actually the Cromium in the metal which keeps it from corroding. Any steel with greater than 10.7% Cromium is classified stainless. You will find that most stainless grades will not rust whilst in the water as rust generally works better in air however, anything less that marine grade 316 stainless steel will be vulnerable to corrosion and pitting. If you do want to use any stainless hardware make sure that it is 316 grade, anything else and you're simply wasting your money.
i've replaced all my external plastic piping with stainless tubing, and there is no rust what so ever, but that's only cos i've got heaps of it lying around at work - shhh