Neither. Even if I weren't an atheist, I'd never be able to accept either of those characters to be the personification of evil. The primary source material has been clearly doctored, mistranslated, misinterpreted, and skewered to fit the metaphors, biases, opinions, and dogmas of many people throughout many generations. Were I someone who put my faith in scripture, I'd call the scripture inherently flawed, and unworthy of my faith. It's with that in mind that I would call humans the "prime evil". As well as the "prime good".