Aliens, defininetly aliens. Sure I fear death, but yautja can show mercy, they will probably let you live if you are unarmed, and since I know of thier code of honour, I could probably negotiate my way out, so I am not afraid of Predators at all.
Aliens, however, freak the living crap out of me. They know no mercy, and if they get thier hands on you, you're going to suffer the worst fate possible. Being ripped apart from within... Not the way I'd want to go. Not only can they get you with facehuggers, but their full adult forms are living nightmares.
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To be powerful, you need an army.
To have an army, you need to be powerful.