Boy, I dunno. I'd be careful about saying one is harder or easier than the other. Both are incredibly different skill sets, and require completely different ways of thinking about and organizing information. Both are work.
For example, I consider fiction writing incredibly difficult, largely because I'm not a good plotter. I mean, forget a plot twist, I'd be happy just to be able to plot. Yet, I have a pal who practically bleeds plot -- he can be inspired by a turn of phrase, a piece of music, or a broken piece of machinery. To him, fiction is "easy" (in the sense that any writing is easy), and non-fiction is like writing a term paper. He'd rather submit to waterboarding than have to do that sort of research and writing.
But let me turn it around now and talk about plotting in a different way, so you can see why some fiction writers might argue that fiction is "easier on the brain" than non-fiction: in fiction, you're creating a world and plots and characters and dialogue -- that's hard work, but ultimately, you control the reality, it doesn't control you -- and that can be very liberating. In non-fiction, though, you don't have the luxury of getting your "main character" out of a jam creatively. The world can't become the way you want it, the ending can't conform to your idea of a "good ending". You have to live within the confines of What Really Happened, which can sometimes be a bitch.
For example, if I'm writing about Alexander Hamilton, I HAVE to have him killed in a duel. Even if it would be cooler to have him somehow survive -- or have him die in a sword fight instead -- I can't do that. George Washington can't use a landspeeder, and the Beatles have to break up, no matter how much I might want things to come out differently.
Now, to me, that makes it easier than fiction simply because I don't have to plot it -- yet, a fiction writer might be tearing her hair out because she's being confined to the unbreakable structure of the real-life plot. For a fiction writer, that can be the equivalent of a straight jacket. Creating the world would be far more preferable -- and, to her, probably "easier on the brain." But that's why that's her talent.
Writing non-fiction and fiction require different kinds of super powers. I once joked that I'm not a Good Plotter, I'm a Good Explainer. To me, non-fiction IS easier than fiction -- but that's because I can't do what fiction writers do. But some of my fiction-writing friends are the other way around. They think non-fiction would be impossible (or, at the very least, unenjoyable) to write.
We all have different skills and super powers.