Stainless is preferred for longevity. But it's definitely noisier than a mild steel system. Things are moving toward full S/S though, so you pays your money & you takes your choice. I think it costs roughly the same to have a system custom made from mild steel or stainless. Personally, I'd prefer to go after mild steel items, either NOS or good second-hand for as long as I can but as stated, they're getting fewer & fewer.