alien all the way. Here is why:
the aliens will just take a predator to be cocooned, and make a more powerful version of the alien or the predator. so the alien gets my vote.
alien is just a superior species. Remember in the second alien movie, called "aliens", when the humans are getting their asses kicked, and they retreat to a compound, then the aliens cut the power, and the humans are like "what do you mean THEY cut the power?!? how can they cut the power?"
If one of those aliens got onto earth, we would be so hosed. But the predator has already come to earth and he got his ass kicked. If the alien came to earth, they would take over pretty quickly.