I do both, or three things, really. I have some fanfic I've been working on for years and a new project, a het romance novel because I'd like to make a living at writing eventually, and my "real" original work that's never going to make me a living. I get burned out if I work on one thing exclusively, especially if there's some degree of stress attached to it, like the first story I published. The fanfic keeps me from getting bored silly by the novel, frankly, because I just can't get used to writing about relatively normal people in relatively mundane settings, lol. If you can only focus effectively on one project at a time, that's one thing, but it doesn't have to be either-or.