The American Film Institute's Top 100 Greatest Horror Movies may have Alien ranked at #6, but consider some of the other films also included in that same list...
#8. The French Connection Really?
#10. Raiders of the Lost Ark Yeah, sure.
#11. The Godfather WTF!!!
#19. The Great Escape Hahaha!!!
#23. Lawrence of Arabia Now they are just taking the piss...
#25. Titanic Actually this was truly horrific, just not in the right way... LOL!
#27. Star Wars Pffffft!!!!
#37. Casablanca Priceless!!!
#39. Die Hard ROFL!!!
Somehow, I don't think the AFI's list is particularly accurate... LOL! LOL!
Well according to Time Out, Alien is ranked #5 in their list of best Horror films.
However, Alien is ranked #3 in Time Out's Greatest Sci-Fi films of all time (just behind Blade Runner and 2001: A Space Odyssey), so I'm assuming those same "experts" believe Alien to be a better Sci-Fi than a Horror movie...
http://www.timeout.com/london/film/the-100-best-sci-fi-movies-10-1?package_page=45597