Both were very different creatures, invented by a society that was changing from the naieve, hopeful race we one were into the jaded, money-grabbing bunch we are now. Each feeds off our fears, and at the same time on our ambitions.
The Xenomorph is obviously a biological weapon. It was created by something to be perfect, and in many ways it is. It is not bound by the constraints of petty morality - it does whatever it must to survive. Physically, there's no match for it, and those who under-estimate its shocking intelligence are prone to get face-raped.
It preyed on the idea that we might be so ridiculously out-evolved that we'd become nothing but food, that as strong as humanity has become, we're really still helpless. Alien and Alien 3 captured that perfectly. Aliens, well, that was born from the Veitnam war, with the hyper-advanced Americans being laid low by the low-tech Veitnamese.
I'm still in denial about Alien Ressurection. Fucking newborn.
As for the Preds? In a way, they are a parody of us, of the 'Big Game Hunters' you see in Africa, Britain and America. They don't regard their prey as equals or accord them any rights, so they have no qualms in brutally murdering them for sport - like us. However, their entire society is based on it.
Preys on the fear that, once again, we're not on the top of the food chain, but that something out there -is- hunting us. It wasn't until AvP that the Predator was accorded some kind of 'nobility', in the first movie it only spared the unarmed because it was no FUN.
Pretty terrifying, and whenever I play an AvP game I know which I am more scared of (the Preds), but the award has to go to the Xenomorphs here.
Why? Because they are not a rip-off of our own species, but something entirely new - something completely unlike ourselves. It is the fact we can never understand them that makes them truly scary.