So let me tell you what I did: I wrote a novel first. I've always had novels as my ultimate goal, and it honestly never occurred to me to write short fiction. I didn't read it. I had no familiarity with the markets.
But James Gunn was the one who suggested to me that I work on publishing short fiction first. Why? So that I would learn to structure my stories better. One of the best things about writing short fiction is that you must learn to economize. That meandering subplot? No room for it. That extensive backstory for character X? Nope, no way to stick that in. You have to learn to set those aside.
I tried writing short fiction while I wrote another novel in my very limited spare time, sent it to Critters, got lots of input. I considered the commentary. (BTW, I don't think I've ever published anything I sent to Critters. Weird, huh?) Then I tried short fiction again, concentrating on getting it right. (I'm no good at reading 'how to' books, so it's pretty much trial and error for me.)
And I began to submit....and then to sell.
For me, the short fiction method worked pretty well. It taught me how to keep my novels concise. My first novel was 153K where nothing happened in the first half*. The novels I've sold--about 110K. I've slimmed down drastically (unfortunately, I only mean that in terms of novels.)
Other people have been successful doing only novels, but for me I didn't have any luck with agents until I had some writing credits under my belt. I'm not a great query writer (in fact, my query letters are pretty terrible) so I think I needed the sales to prove that I am professional. That doesn't apply to everyone.
But for me, having short fiction under my belt gave me both credits and experience structuring stories. It was the correct call for me. ;o)
What do you think of chosing the short fiction route?
And here, BTW, is Elizabeth Wein's excellent commentary On Reviewing. I find I agree with everything she says there....
*I know how to fix this novel now, but I don't have time...yet.