Great question. They're both enjoyable - and challenging - in different ways. Writing in someone else's universe is easier, but more restricting, because you've got the structure pretty much set up for you. Don't have to go into pages of explanation about the technology, because everyone understands transporters, warp drive, replicators. Don't have to go into long-winded descriptions of the established characters; you can get away with a few brush strokes. What's restricting is the fact that you have to Put Everything Back Where You Found It. The simplest example of that is that Edith Keeler had to die because Kirk can't be saddled with a relationship that's going to distract him from his One True Love. Those restrictions have been loosened considerably in the past decade or so, but there's still always a framework and a timeline that has to be adhered to. In creating one's own universes, there are no such limits, but it's a lot more work. You literally have to build a society from the ground up, and think everything through - the tech, what your "people" look and sound like, what their culture and philosophies and myths are - as a species. Only then can you get into different characters, how they're distinct from each other, and what drives them into the conflicts or crises that move the story along. In your own universe, you also have to concern yourself with the size of your story. Is it a single, self-contained novel or even a short, or is it an three-book epic or an open-ended series? That, of course, depends on what your editor's buying, but sometimes what starts out as a single novel can get away from you and grow into a trilogy. So those are the challenges of creating one's own universe. The enjoyment comes from sometimes literally not knowing what's going to be on the next page. Sure, you've got an overall idea of where the story's going, but sometimes you pull a sentence or a transition out of the air and go, "Huh? WTF did *that* come from?" and it takes you in an entirely unexpected direction. That's not to say that doesn't happen when you're in other people's universes as well.