I always write what I enjoy, otherwise it seems like a chore. Prior to writing 'Gone Away, Gone Ahead' I'd never ventured into fan fiction, although I'd read quite a lot of it. I find writing fanfic is more challenging, because you have to try and borrow another writer's voice and keep it true to the world you are writing in. There's a lot of going back to read the originals to get timelines right and to make sure canon characters seem authentic, even if you are exploring aspects of their personality that aren't covered in the original.
Writing original fiction is easier in a lot of ways because you are designing your own world, its rules and characters from scratch. It doesn't give you a ready made audience, so you have to hook them early on. A lot of my original works are based around what I know about or am interested in.
Years ago, another writer and I tried to go down the commercial road by writing women's romance fiction (we were in it only for the money) but we discovered that you still need to take the genre seriously. Our characters soon diverged from the set formula and we ended up with something that didn't really fit into the niche and unsurprisingly, didn't get published. That was when I realised how much hard work writing can be when you are trying to complete something and your heart isn't really in it.