Novels and short stories are similar in many ways. They both tell a story, usually involving characters and plot. They both entertain, engage, or educate the reader. And they are both written by a (hopefully) diligent writer.
But writing a novel is simply not the same as writing a short story, and it's not all about length. I will be the first to admit that many of the differences between novels and short stories are related to length, but it's still worth looking at these differences.
The first difference is in the amount of material that can be presented in a novel. A short story is under 10,000 words. Any longer than that and it should be more accurately termed a novella. With only 10,000 (probably less) words to work with, you have to be selective. You can't have hundreds of characters and twelve different subplots. You simply don't have the room. So, writing short stories means you'll have to learn how to choose what's important to the story itself. Your plot and your characters will be limited. In essence, a short story ask for economy of the writer.
In contrast, novels have really no upper word count. If it's over 80,000 words, it's definitely a novel. But you could write as many as 500,000 words. That's a lot of space to do whatever you like. You can have a thousand characters and multiple plots and an infinite number of subplots as you craft your novel. A novelist can afford to create new problems and obstacles for characters to solve simply because there is room to solve those problems. As long as the core of the story remains intact, and the readers don't lose sight of this core for very long, the novelist is free to move about and create according to whatever whim happens to appear. Not every word in a novel have to mean something. Words can exist in a novel purely because the writer wants the reader to experience them. As long as the readers aren't lost, it works.
Short story writers don't have that luxury. If a short story deviates from its core, even if it's just for 1000 words, it's very hard to recover. Deviations from the core tend to be edited out or survive as a weakness in the story. Short story writers have to economize, so every word has to count. Not a single word in a short story can exist just because the writer wants it there. Each word must serve a purpose.
Which is more difficult? Both have their challenges. Short stories demand economy and novels demand that the writer be in it for the long haul. They are both valid and beautiful art forms. Pick the medium that best fits your story and craft it well.
