The similarities are many and the differences between Stephen King and Dean Koontz are hard to find for some because they both write in the same genre, that being horror and suspense fantasy.
How popular are Stephen King and Dean Koontz? Each has had novels on the New York Times Best Sellers List, King having 28 on the Number One Best Sellers List, some of which are "The Dead Zone," "Misery" and "Dolores Claiborne." Koontz having 10 books in total, on the Number One Sellers List, some of which are "Hideaway," "Intensity" and "Midnight." Some of these books have spent up to 14 weeks or longer on the Number One Best Sellers List.
Both authors started out writing short stories for different men's magazines. Both have had movies based from some of their books; some have been made for either TV, like King's "IT," or Koontz's "Mr. Murder" or a mini-series like King's "Kingdom Hospital," or Koontz's "Frankenstein," or even for a major motion picture like King's "The Secret Window" or Koontz's "Phantoms." Both authors' are now reaching out to new audiences by ways of graphic manga novels such as Koontz's Odd Tomas, and King's Gunslinger, comic book series.
I have read many books by both authors and I have found that the styles of each author do differ, the first being that King writes with more fantasy fiction such as "Pet Sematary," (where a kid comes back to life after being buried in a pet cemetery) or science fiction point of view like "The Tommy Knockers" and "Dreamcatcher" (Where aliens come to earth to conquer and kill humans.) Whereas Koontz writes with more of a fiction with possibilities, point of view such as "Intensity" or "Watchers." (Where science projects go astray, killing everyone in sight.)
On the subject of horror books, my opinion of the writing styles by King and Koontz is that they are distinctly different. King's style screams that this will scare you by its blatant use of graphic detail. (e.g. The Mangler, Cujo). Whereas Koontz's style is subtle and to me says if it does not scare you the first time, you should dim the lights and read it again (e.g. The Servants of Twilight, Whispers.) Both authors are drawn to the occult King's "Thinner" Koontz's "Corner of his eye" in which both stories deal with black magic, curses and hexes. In the end both authors show how good prevails over evil. "The Stand" by King or Koontz's "Warlock"