Hmm..
Stephen King
Dean Koontz

I have read all the books from both writers, and I find a couple of differences..

Dean Koontz:
I find him a good writer. Not very detailed or burdened with facts. His books are a quick, fast read and very entertaining. After finishing one, I am ready for another book of any kind...

Stephen King:
I find him an excellant writer. Extremely detailed. You have to be sure to read every word or else you might possibly miss some very important detail that leaves wondering what ya missed. you then read back to fill in the gaps. Takes much longer to read due to the detail. Very facinating stories and ideas. I sure would hate to have his nightmares... After reading one of his books, I can go weeks trying to find something else good to read because of his style and content... On a downside, since I have children, his leaning towards the transgressions done to children have left me somewhat cold....

Movie...
I have yet to see a movie that has done justice to Stephen King stories. Very few come to mind that were actually close to the original stories. And those were short stories at that...

Koontz's lighter style of writing would be easier to translate to screen I think. Of course not all of the would.

Disclaimer: This is just my humble opinion...