I dunno. At the very least, you'd have to call Alien a hybrid horror/sci-fi movie (maybe the first ever of it's kind). Aliens, the sequel is more properly a classic sci-fi action flick. Alien 3 returned to the horror aspect.
You have no idea how many arguments I've gotten into work about how good Alien 3 actually is. People just can't see past the (most excellent) action bonanza of Aliens.