I read a few of each in High school. Koontz was fun but to me felt a bit more targeted for mass appeal (as weird as that feels to say in comparison to King). Then again, for King I was reading stuff like The Stand and The Dark Tower series, which may or may not be indicative of the rest of his work (which I mostly know through the movies made from them)?

I recommend checking out one of King's novella or short-story collections for a different perspective. Some of those are actually some of my all time favorites.

"Different Seasons" is my favourite collection, and out of the 4 total stories in it, 3 became movies (The Body became Stand By Me, Apt Pupil, and The Shawshank Redemption). Can't recommend this collection highly enough.

I read King and Koontz during my teens, my two favourite books of each author are 'It' by King and 'Phantoms' by Koontz, I actually read 'It' first and thought Koontz had been inspired by King given how they were quite similar, however I learned later that Koontz Phantoms was published in 1983, while It was published in 1986, so I guess that if anyone was inspired by the other it was King.

Never gave Koontz a try. Frank Herbert and Ray Bradbury were my other favorites. I tried a few other "popular" writers (Grisham, Agatha Christie) but thet didn't really make an impact. Do you recommend Koontz?

I read some Koontz around the same time period, but honestly King had so much work that I didn't find a good reason to continue doing so.