I'd go copper. I've HEARD that SS is a real pain to bend without kinking. Also, I'm no chemist, but I've read that copper ions help when they're in the boil.
Here's the article in BYO about metals in brewing. Never put copper in the fermentation.
and here's the quote about copper in the boil
Copper is a double-edged sword in brewing. It is beneficial before fermentation, but detrimental afterwards. Copper ions react with the hydrogen sulfide produced during fermentation and reduce it to insoluble copper sulfide, which is left behind with the trub and yeast cake. Switching to all stainless steel brewing equipment can lead to noticeable quantities of hydrogen sulfide and sulfur off-flavors and aromas in the beer. The use of copper wort chillers will provide all the copper necessary, as will including a short piece (1 inch) of copper tubing in the boil.