Looks like I didn't explain enough to get my original question answered, so I decided to answer the questions you asked in answer to the question I asked. Ha!

My thinking went like this: if I like a movie that is an adaptation, I want to read the book. If I like a book, I look for an adaptation. So they are mutually promoting, and this was my motivation for pursuing this course. My original question was about maximum impact--which way would result in the highest mutual promotion?

I agree that adaptations are hard and frequently poor, but I thought as the original writer in both media, I would be in a unique position of seeing what was necessary more clearly. Hence I could make both stories powerful.

I think studying some adaptations can be useful. Twilight, the book, relied a great deal on the characters' thoughts to create suspense, which the movie left out. Harry Potter/Philosopher's Stone tried to follow the book too closely, thus turning exciting and original writing into scenes that dragged and looked predictable. I read Patterson's book after watching Along Came a Spider only to find out they were completely different stories.

What else can we learn?

I think the real question is, which medium interests you more? If you really want to be a screenwriter, and you know your story, just write the screenplay. Writing the novel will take away a lot of valuable writing time that you could be using to polish the script or move on to other projects.

If novel writing interests you more, forget about the screenplay for the time being and focus on making that novel a success. You don't want to be constrained by the vastly limiting set of principles that apply in the film world. Novels are about exploring the inner workings of the mind; movies are all about what can be expressed visually. There's a very good reason why so many adaptations turn out to be failures, even when adapted by incredibly talented writers and directors.

Writing both, either concurrently or one after the other, simply as a marketing gambit, doesn't strike me as the best use of your time or creative energy.

I took this exact route to flesh out a story I want dramatized.

Looking for maximum impact? Write the novel first and self-publish it as an eBook. You don't have to wait for it to be picked up by a publishing house.

My novel's already selling; and I'm sending comp copies to interested individuals and affinity groups. The resulting online awareness for it earned me the attention of development execs from three different production companies, two of whom hang their shingles at major studios.

The D-folks are always scouring the publishing industry for a book manuscript with a workable premise. The proliferation of eBook distribution websites has just given them another venue to search for material.

The company that purchases my novel's film/TV rights gets to see my screenplay adaptation. That should provide a rolling start to the process of attaching elements. I can't be sure of the degree of influence this will provide me (yet), but I'm sure that it will be a better negotiating position than if I went in with only a screenplay.

The extra time spent on the novel was definitely worth it.

My favorite adaptation is Bleak House by Charles Dickens, recently turned into a mini-series by Andrew Davies. I loved how I could follow the story despite its being 8 hours long and chock full of complexities and characters.

Favorite adaptations of novels?

Hm. Kubrick has a few: DR. STRANGELOVE, A CLOCKWORK ORANGE, BARRY LYNDON (I don't care for LOLITA, though; Nabokov's screenplay was far superior to what Kubrick put on screen). Coppola's APOCALYPSE NOW is a magnificent version of Conrad's HEART OF DARKNESS.