Ehh.... I dunno.
I've been a big fan of both but I tend to lean towards Koontz.
One thing that bugs me about King's writings is that it often seems.... unnecessarily long-winded. Usually when characters are talking. Like, alot of bits that make me go "what does this have to do with anything?"
Something I noticed with some of his books was that I could take the book, flip it open to a random page, and instead of something happening that was actually related to the core story or actual character development/progression, it'd be someone rambling on about nothing. "Yeah, see, my granny now, she had a FACE. With eyes. Old people, they all have faces." And then there'd be like this 5 page rant about how his granny earned that face of hers. And it's like.... okay... great. WHY are you talking about this so much? Can we get back to the.... whatever was even happening? The pacing overall tends to be very.... slow. Too slow. His larger books arent larger because they need to be, but moreso because they're overly padded. Of course this depends on the individual book. For some he's alot better about that, for others I cant even get through the bloody things.
Not that he's the only author to do that. HP Lovecraft does this one alot. He has a tendency to over-describe things like locations and objects, so certain parts of his works can get oddly bloated for no reason. I find it works better for him though because all of his stories are inherently short to begin with. Even when they get a bit padded, they dont even come close to overstaying their welcome. He's not writing that 1200 page book where 900 of those pages are about how ol' grandpa earned his elbows.
Koontz seems alot more focused overall, though it depends on which of his books it is. I'm not as familiar with his oldest books, so those might be different for all I know. But typically with his, the story keeps blazing on.
Not that he doesnt have his issues. My friends also read his books, and one funny thing we noticed (doesnt happen as much in more recent books) was what I always called the "Koontz Ending". Where some villain or something would get built up over the course of the book, and then when you expect a big confrontation, a tree falls on him and he goes splat and that's it. So whenever that happened in any book or TV show or anime or whatever, "yeah, that one has a Koontz ending". Granted those moments usually arent the actual end of the story, but moreso the end of that specific villain. Only happens in some of his books, but still. On the other hand there's certain books where the final confrontation is unusually epic or the villain has an unusually horrific end. I seem to recall one villain that got teleport-spammed until he was nothing but this literal heap of organic material and rat corpses. Granted, he kinda had it coming.
Also, yes, there is always a dog. Always.