Nonfiction because: my faulty brain sometimes mistakes fiction for reality(e.g., I used to believe that Santa is real) and cognitive economy - there is a finite amount of knowledge I can store, so I would rather make sure it's accurate, truthful, useful knowledge.
tomme, welcome to lesswrong, gday I'm Peacewise.
re
I used to believe that Santa is real
Fair crack mate, "Santa" is a standard fiction/lie perpetrated by society and parents, hardly something to be used as evidence of a "faulty brain". In fact its more likely to be evidence that your brain was and is functioning in a developmentally normal state.
I suggest you reconsider your position on fiction, since you state
so I would rather make sure it's accurate, truthful, useful knowledge
there is indeed plenty of accurate, truthful and us...