This thread should have been set up as a poll question. It would be cool to see how people on the forum lean. I like horror and I like sci-fi, but horror and sci-fi together, I love!
It is my opinion that sci-fi is more story and fantastical setting oriented, while horror is often more about the suspense, dread , horrific moments about an incident or character. This is why I really like the combination of the two genres - I get my thrills and a story to go with it.
I would use a lot of slasher movies as an example of formula over story - kids have sex and get killed and the "good" character defeats the evil. This is not true of all horror, but it is a genre that can get away from tradition, because it is so visceral.