Alien for me. I like the movies better, it has an interesting design, interesting life cycle, adapts to any situation, crawls on any surface, adapts traits to whatever it seeds, doesn't give off body heat, has ACID FOR BLOOD, a freaking huge tail that can break almost anything, can see in any light, and has a mouth inside of his mouth that can break almost anything.
Also, a Predator going up against humans and Aliens at the same time is screwed. They both require their own separate vision mode. And Predators are hard of hearing. They're hearing is distorted with the mask on, and with it off, everyone sounds like chipmunks.