Look at the rankings. Fiction sells better but its harder to wirte fiction books because you have to be creative and have to know how to write good stories. It takes time to become an authority person (whats important in the fiction area to be successfull). If you have talent then try it.
On the other hand: non-fiction is easy to write. Its only a research work and every house wive could do it (so you can outsource the writing work). Your major work is to analyse the store, finding niches and keywords and market your books. And its also easier to build a business around a non ficton niche (other products like smartphone apps, audiobooks, videos, websites and so on). So If you are not a creative person, non fiction could be the better choice for you. But it doesnt let you make so much money like ficton books.
I prefer non fiction because i am better eg. in analyzing the market things, handle the outsourcing part, thinking about other products and so on. It only depends on your personality which way you want to go.