To be honest it really depends on the situation in which both species are placed in. The Alien would probably get my vote just because it's immune to pain, can lose limbs (can probably regenerate them as well), and hell even lose half of its torso and still keep coming. For the Predator, pain to them is just common place, and dampened through all the rough and sever years of trying to come of age. If I had to best describe the Predator's society with another, I think their society resembles more closely to that of Sparta, but in the more simple since of how they go about their business in bring up their warriors. But as I said before it really depends on the situation, because the Alien has no chance against a heavily armed/armored Pred. As a audience, we only have tasted mere glimpses of what kind of weapons the Pred's use in terms of hunting sport, and one could only imagine what they pack when they're about to do war. A lot people talk about the scene in Alien 3, where it is emerged in hot lead, and is still able to survive, but those same people hardly talk about what they saw in Aliens. Even though the Marines got their assess kicked in that movie, they still managed to kill a good number of the Aliens with what little that had in man power, and supplies. But in Alien 3, the Alien itself was in an enviorment where it essentially wasn't threatened. I don't think any of the prisoners were going to kill it will spoons or forks, and besides you heard everyone ranting about, "lets wait until the military hardware gets here". Anyway in a general since, I would give my vote to the Alien.
