I don't want this to be a pissing match about which is better. I just want to know why you like one better than the other. By the way they are both awesome monster in their own way.
I'll start.
I'm a Pred guy.
I like humanoid-like monsters overall. "Creature's" second which is what I would personally classify the Alien.
Predator has behaviors that I find interesting and scary. His size and thought process on screen are more tangible to me. I can "picture" it better to think of if I actually saw a Pred I feel I have a grasp of him better overall and my imagination is more apt to it.
I love his technology, I love his "stalking" tactics. I love the noises and sounds that come from his kind (great job by McTerian and his staff with the "noises"), his "look" I think is classic, scary, intimindating etc... All the things that trip my personal imagination.
The Pred movies to me are more about being scared and hunted by the Pred. You feel more helpless against them, which is cool.
The Alien movies which I love (1, 3- great 3,4- cool) are more about you hunting the creature, seeking it out after you discover it (especially 2-4), is how I feel about them.
I love that the Pred is mysterious and nobody really wants to "know" more about it, merely survive it's wrath, where the Aliens were actually sought to be captured and studied.
Two different "feels" to each monster.
Both awesome.
But I like Pred the best.