I had this idea for a novel, so I started writing in January. I wrote 40k words or something like that and then realized I had no idea what I was doing. The more I read blogs and articles on writing, the more I ran into concepts that I didn't really understand. Character arc? Setting? Rising action? Different kinds of conflict?
In that research process, I discovered this contest. $1000 says you? Alrighty, says I. So I took a break from the novel in March and wrote a short story. In the process I learned a TON about writing. Character, Plot, Setting, Finishing, etc. Then I had to edit the darn thing, which was an entirely different process than writing but just as educational. I submitted it with grand ideas of many accolades

and turned back to the novel. Lo and Behold, writing the novel felt more solid now. So when the deadline for Q3 started to loom, I took another break from the novel and wrote another short story. Same with editing and my Q4. I'm currently querying agents for the novel, working on my Q1 story, plotting my next novel, and brainstorming for my Q2. All in all, I've written 5 short stories (two of which will never again see the light of day) and three that I think have some real potential.
My point is, writing short stories informs the novel writing process (at least for me) in a big way. It is NOT the same skill, as with shorts you have to condense and refine a great deal whereas with novels I think you're given more space to expound and readers expect more detail. But they are related skills and, at least for me, doing one significantly improved my ability to do the other- especially with the editing process, which with a novel can feel overwhelming.
If the story demands that it is novel length, trust your story. I guess I don't think the two need be mutually exclusive- you can write a novel AND write short stories, and you'll be better at both for the diversity.