Loce this quote, courtesy of David Foster Wallace in "Deciderization 2007 -- A Special Report" (this pretty much nails it as far as non-fiction is concerned; I would not know about fiction!):
"Writing-wise, fiction is scarier, but nonfiction is harder—because nonfiction’s based in reality, and today’s felt reality is overwhelmingly, circuit-blowingly huge and complex. Whereas fiction comes out of nothing. Actually, so wait: the truth is that both genres are scary; both feel like they’re executed on tightropes, over abysses—it’s the abysses that are different. Fiction’s abyss is silence, nada. Whereas nonfiction’s abyss is Total Noise, the seething static of every particular thing and experience, and one’s total freedom of infinite choice about what to choose to attend to and represent and connect, and how, and why, etc."