Well, compare "Alien" and "Predator" and you have two VERY different movies. Yes, both of them have aliens/extra terrestrials in them, but that's about it.
"Predator" is a basic action movie with big muscular men with big guns kicking drug dealing/guerrilla soldier ass in the jungle, just to end up getting their own muscular asses kicked by one single alien that is even more strapping and packing then them. This is a great premise for a movie (hunter (=drug dealers/criminals) gets hunted by a bigger and stronger hunter (=US elite soldiers) who gets hunted by an even bigger and stronger hunter (=the predator)... and so on and so forth). Even so, the movie is executed like an ordinary action movie -- one-liners, macho bullshit, orgies in guns & guts etc.
"Alien" is a basic sci-fi space movie with a hapless Average Al spaceship crew without any guns (ok, they have a few cattle prods and homemade flamethrowers) finding an alien derelict from where they bring back a nightmarish creature that stalks and kills everyone on board. The plot of "Alien" is actually more basic and B-movie-like than the one of "Predator", at least in my eyes, but the way they made the movie turned "Alien" into something way bigger than yet another sci-fi monster movie. It has great characters with great dialog performed by top-class actors (Harry Dean Stanton, John Hurt, Sigorney Weaver, Tom Skeritt, Yaphet Kotto etc.); "Alien" is character-driven - it's all about psychology, terror, behavior and the human psyche. The "Alien" itself is not designed to be cool just for the sake of being cool (which I believe is the case when it comes to the Predator - and the Predator is indeed flawlessly cool and bad-ass beyond comparison *not sarcastic*) -- the alien is art. H.R. Giger, the surrealist artist, created the creature from the basis of his artwork and the theories surrounding Surrealism (such as idea about the unconscious presented by Freud, or the ideas about the Shadow presented by Jung), hence making the Alien creature into something that dwells beyond regular 'coolness' and conventional monster design.
They're both awesome and great in their own way. I mean, "Alien" is a worthless action movie just as "Predator" is a worthless sci-fi film (with the term "sci-fi film" (and not "sci-fi movie") I refer to classic films such as "2001", "Solaris" (the original), "THX" etc.). What I'm trying to say is that if I want to be entertained, as well as gorge in raw sweaty coolness, I'd rather watch "Predator" than "Alien"... And if I want to enjoy great film making, claustrophobic darkness as well as analyzing things, I'd pick "Alien" over "Predator" any time any day. When I crave steak I want a bloody steak and not sushi, and when I want sushi I want fresh delicious sushi and not steak.
"Predator" is a great sci-fi action MOVIE.
"Alien" is a great sci-fi horror FILM.
...In the end they're both great at what they're doing.
