The Aliens attack in packs, and can crawl and hide in hard to reach places. I'm not sure they emit a heat signature, so I wonder if the Predators' infra-red will be any good; perhaps that's why the ALien species is chosen as the Predators' initiation ceremony, because they pose such a challenge. And the ALiens don't have eyes because they sense their surroundings differently, by sensing changes in air pressure, vibrations in the ground and air, etc. Just look at the second ALien movie, where they lie dormant in the walls until food comes along, and then they are up again. Same thing goes for the facehuggers, in the first Alien movie they wait in their eggs until a host approaches and they can wait there indefinitely. The ALien species can survive for indefinite amounts of time without food; they have adapted to every environment they may find themselves in, which is why they are so successful throughout the universe.
One on one the Predators are better at combat, but they are big and clumsy and need technology to win. Stripped of their weapons, the aliens would destroy them, not only because of their acid blood but because of their relentless instinct to kill first and ask questions later. The ALiens are uncivilized insects and the Predators are civilized warriors. The battle is almost perfectly matched, which is why this question of who will win is pointless.
You know, like how you used to do as a kid? 
