Zinc naphthenate vs. copper naphthenate

When I bought some siding the other day, the receipt had a note printed on it advising me to treat all cut edges with a "zinc naphthenate-type" product.
I'm curious, since I've heard of and used copper naphthenate (aka "copper green"), but never heard of the zinc compound. What's the diff? Is one better? Better for certain situations?
I like using copper green; the first time I encountered it, I thought it must be terribly toxic stuff, because of the smell and the name of the compound, but after checking the MSDS found that it's actually pretty safe. And I actually like the smell.
Hmm; just looked it up, and zinc n. seems to be much more toxic than copper n. Compare http://www.pesticideinfo.org/Detail_Chemical.jsp?Rec_Id=PC34736#Toxicity (zinc) with http://www.pesticideinfo.org/Detail_Chemical.jsp?Rec_Id=PC35068#Toxicity (copper). Zinc n. is listed as a "PAN Bad Actor Chemical" and as a "Developmental or Reproductive Toxin", where copper n. is neither.
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