I agree with alex. Writing a short story is a lot quicker than writing a novel, and so is easier in that sense, but writing a short story that a professional publication will accept isn't of itself an easy option.
I know you take part in the Challenges, Ian, so you're getting some experience in writing a complete tale, but do you feel you're progressing there? Are you getting many votes? More votes on average now than when you started, especially in the 300s? If not, have you analysed why? More particularly, have you asked for feedback in the Improving threads? The quickest way to progress as a writer -- if, regrettably, also the most painful way -- is to get someone else to critique our work, so we can see what we're doing wrong and how to put it right. This is a vital step if you want to be a professional writer. As Alex suggests, you could go into Critiques if you write a longer story or two by way of practice, but don't forget that you're unlikely to be able to sell a story if it's appeared there in any form, so using the Improving thread for 300s you've already written is perhaps a safer, and certainly more speedy, way to begin.