Jennifer Egan writes both. I love her take on the difference:
There is nothing the same about them. For nonfiction, the writing part is almost an afterthought. With nonfiction, it is basically the job of synthesizing a gigantic amount of information and experience into something crystalline and relatively short, although my pieces are relatively long. The Lorie Berenson piece took a really long time to write, I really struggled with the writing. But some of the ones before that I wrote in four or five days.
In fiction I am doing something entirely different— I am letting it rip in an almost unconscious state to see what I come up with, and then I decide what to do with it.
With nonfiction, I am dealing with the world. There is a kind of great feeling that happens with nonfiction, this sense of clarity about a subject and an excitement about sharing that in all of its nuances. Once I reach that point, the writing is easy. In fiction it is precisely the opposite because it is the act of writing that generates the material and then it is a process of years before I have really processed that and turned it into something interesting.