For fictional, you have freedom to make them relatable however you please. There's no real limit to how you have your characters grow. As for nonfiction, you're a bit more limited to what you can do to make them true to their real life counterparts. If you start to delve into something that's out of character for the real life person, people will be upset. But at the same time it's difficult to capture someone's life without knowing them on a deep personal level, but even then that doesn't capture them truly.
So basically you have more freedom to flesh out characters in fiction while in nonfiction it's difficult to capture what they truly were while making them an interesting person to relate to. This is what I've noticed between the two, I usually enjoy fiction more cause I like escaping to another world and that's about it