The art of writing a short story is distinctly different from writing a novel. There is far less “room” in a short story than a novel; you must make every word count, particularly if you are up against a tight word count. The story must be unified by a single theme or over-arching idea; there is no room for subplots and parallel memes. Depending on the genre, the notion of a twist is also quite prevalent. In fantasy literature, for example, there must be something in the story that takes it an unexpected direction, or flips our expectations upon their head. A piece of clean prose that tells a simple straightforward tale will not cut it. Nowadays, there is a requisite of something unusual, something that makes one look at the story from a clever angle.
One is also limited by characters and viewpoints in the short story. Generally, the writer is limited to a single viewpoint with but one protagonist and one antagonist. There might be a couple other minor characters, but they will be few in number and their roles hardly substantive.
A novel is an altogether different animal. While it is true, that there is usually a central theme for any great literary work, there also can be sub-themes and sub-plots and what have you. These can be developed throughout the course of the work, because there is no word-limit on a novel (although length of a novel certainly is no indicator of quality). Complex literary devices like symbolism and such can be fully employed in a novel; one has plenty of room to develop and expand upon such concepts. In the fantasy genre, the novel, like the short story, must be more than a straightforward tale. We are drawn to the unusual and the surprising. Because of its greater length, the novel has room for multiple twists, each one eliciting a pleasant burst of “ahh!” from the reader. A story of a brave knight rescuing a princess from a dragon no longer cuts it today.
In the fantasy novel, multiple viewpoints and plotlines have become almost standard practice. Rare is the novel with but a single protagonist these days.
So which one will you write? For myself, I’ve gone back and forth. I’ve written a number of short stories, with a few minor publishing successes. However, I got into writing to write novels not short stories. Short stories are great to hone your craft, to figure out the intricacies of fusing dialogue with narrative and what-have-you. And I don’t regret the time I’ve spent on short stories—I even have a number of short story ideas that will probably never come to fruition—but I have decided that I want to focus on novels. Besides, short stories don’t pay enough to support a writer. They exist to fluff the resume. And they are great for honing your craft. But, in my opinion, that is all they can do.