Dances With Wolves-another case where the story existed first as screenplay, then was reworked as a novel. (Though I'm prone to think the screenplay was better).
In my case, I had a 1300 page novel that my agent thought was brilliant, but couldn't sell (precisely for this very reason). I've spent the past several months adapting it into a screenplay because there has been some interest in it as a film and I was asked if I would be willing to write a screenplay.
The thing about writing the screenplay is that it's taught me a lot about how to pare the story down to its essence, how to tighten and improve the overall flow, etc. In other words, the lessons I've learned from doing the screenplay can, I feel, now be applied to a rewrite of the novel, and hopefully with better results. At any rate, I now feel pretty confident that the novel can be pared down to about 600 pages, or roughly about half of the original length.
But without the benefit of writing the screenplay, I don't think I would have been able to do this. In about a month or so, once I've turned the script over to my agent and the producer who is interested in purchasing it, I'm going to be starting the rewrite of the novel.
Of course my case is somewhat different from yours because I had already written the story as a novel
first, then turned it into a screenplay and now will be in turn re-adapting it (if that makes sense) back into a novel. The good part for me is that the hardest work has already been done, so it's just a matter of cutting and adding as needed. I can go back and forth between the two mediums fairly easily so that isn't a problem for me.
But I will caution you not to assume that novels are easier "sells" than screenplays. A first-time novelist is up against just as many odds as a newbie screenwriter. In my case, it was largely because my novel didn't sell that I ended up writing the screenplay (and it looks quite likely that it's the screenplay, rather than the novel, that will end up selling first). The thing I'm trying to say is, don't make assumptions about any medium being an easier "sell" than another because this is, if nothing else, a very strange business, and any predictions about what will or won't sell is really pointless conjecture. It sounds like a cliche' but is true-you have to write for yourself first. I happen to enjoy writing in both mediums, but not everyone does. Some people are born novelists; some are meant to be screenwriters. You may find that you don't enjoy novel writing, but on the other hand, if you start experimenting and find yourself having a blast, then by all means go for it!