I see the pros and cons in writing both fanfiction and your own.
with fanfiction, you already have everything planned out: the characters, setting, personalities etc. So you really only have to pay attention to the plot.
But on the other hand, it can restrict you and be extra careful how you handle these characters. Although, that is for writers who want to keep the characters untouched in a sense. Nothing wrong with switching them around =3
For writing your own stories, it's easy in a sense it's your own world. Like a child's building blocks. It's their own design, creativity, and for their own amusement that they don't have to worry half the time if a character should act this way(unless it's an author selling a book).
The problem here, too, though, is there may be a lot of planning involved, depending what kind of story you do. Sometimes you may feel stuck, or frustrated that the characters are flat.
These don't apply for everyone though. Some find one side easier than the other. For me, I wrote a few stories of my own ideas in the past, but I mostly write fanfiction because I enjoy writing about characters I have looked up to since I was a kid. Like an artist that draws superhero comics, but in literature ^^