Stainless steel's heat transfer sucks. This is pretty much true throughout the various "flavors" that are available. Unless you have space for a larger-than-normally-needed radiator, I would stick to copper tube with aluminum fins.
Chromium is the element that makes steel stainless and it's a protective coating of chromium oxide that is primarily responsible for the surface protection of stainless steel. The myriad other metals that you can find in various stainless are mainly for:
1) Determining physical properties, such as strength, crystal structure (austenite, ferrite, martensite)
2) Modifying the corrosion resistance
3) Determining magnetic properties, if any (mainly from crystalline structure)
4) Determining hardenability
As to the copper/brass/(and bronze) questions, they are all close in the galvanic series because they all have copper as their main ingredient. What corrosion you would have with distilled water would be slow. A touch of a corrosion inhibitor will control that without dropping heat transfer characteristics too much.
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