It's an interesting question OP, and, now that i think of it, one that has cropped up in my thoughts too.
I agree with points from so many of the previous posts - the two forms are not the same - but I also go along with the premise that writing anything should make you a better writer.
I have started out with short stories for three reasons.
First: It is true that it takes less time. I guess that one is obvious.
Second: When I think of great writing, not great story telling ,my favourites have always been short stories. Those ones that hang around with you for hours after you have only spent minutes reading them; those sentences that make you gasp.
Third, and perhaps what I think you are alluding to: I didn't feel that I was "qualified" to write a full length novel.
I have so far finished a book of shorts that goes to 60,000 words. Short novel length, I guess. I am in the editing process of another book of very short stories (2,000 words or less) that clocks in at about 30,000. I have, however, written a novel that is so far at only about 50,000 words, for a story that a good novelist could keep interesting for twice that length, or more. But it's a mess and badly paced and, though I am more than happy with the story and characters, I wonder if I have the talent to stretch my ideas into a new format.
I have received some wonderful advice from a woman who, whilst not a writer herself, takes a keen interest in writing and loves to read. Her comment to me was something along the lines of, "Just write it. Then you will know."
I suppose what i am saying is exactly what she said. Write it.
I wish you all the best with it.