Ummm, Aliens is not a horror movie. Was this some kind of major revelation to you upon re-watching it?
No one, including Cameron, ever claimed it was supposed to be horror. It's a Sci-Fi action flick with some reasonably scary moments thrown in for good measure. The film is one of the few sequels to ever change genre successfully. It feels very much like it's own film yet fully connected to its predecessor.
I also believe it to be the superior film of the two, but I know I am a minority in this and would rather not get into that debate.