Aliens have always been stealthy by nature, they use every inch of their environment to make sure they are not seen, at least not before they strike. In the original Alien, the Xeno was a ghost of death, it was impossible to tell just where it was, and where it would strike next.
Stealth comes naturally to Aliens, it's one of thier many survival instincts.
Predators, on the other hand, rely thier mastery of technology to hide them from thier prey. They are skilled in being where no-one would think to look for them, and unless you see those three red dots on you, It's nigh impossible to tell their even nearby, let alone find out exactly where they are.
They don't bother to hide, because thier cloaks do that for them.
So which do you think is more skilled in the art of stealth? Which do you think would be more able to sneak up on you?
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For a Marine: War is life, and Death is the only true Peace. (I wanna be an Alpha Draconis member, GO MARINES!!!)