Predator because they didn't give me nightmares when I was little. I think I was too young to watch Aliens the first time I saw it and I literally didn't sleep for almost a week. But I saw Predator and was like 'aw yeah, gimme some of that'.
And, also, because the Aliens just... I don't know, they still terrify me. They're amazing to look at but if I was stuck in a room with one it'd have to be a Pred, just because I'd know that if it decided to kill me it would be a quick death instead of some icky chest-bursting agony-filled demise.
Plus, acid blood? That's just wrong. Preds and their dreads all the way, man.