I prefer Aliens. This is why:
The Alien was a work of art. It was the concept-child of Giger, and it carried its own commentaries - largely psychosexual. It's something startlingly original and horrifying. Ultimately, it's got an intensely interesting design that's as alien as its psychology. The references to rape indiscriminate of the victim's sex were also wildly, wildly daring for the time (and would even be daring today).
The Alien, in both concept and practise, had absolutely no restrictions. It was an amazing creature from an amazing movie that carried commentary and underlying, psychological exposition.
It was, and is, art.
The Predator on the other hand is a Hollywood cop-out. It's basically just a big guy with a big gun. The warrior-tribe thing is pretty unimpressive, given that such a culture is directly influenced by cultures already existing on earth. In addition, it appears mostly to appeal to the kind of audience who don't grasp that the point of any good narrative is to explain a concept, or reveal a way of thinking. The Predator movies never set out to do either of these things, and the monster design reflects this. It's lazy, boring and signifies nothing.
The Predator is a great representation of Hollywood money-mongering and it's a shame that the genius of ALIEN ended up being brought low by such tripe.
Uh, sorry to Predator fans, I guess.