Well, saying that I haven't seen a lot about either. (I only watched Alien and Aliens and then I watch Predator) I think Aliens would win on this one. Let the debate begin!
This is not a good topic for a debate. It's about fictional characters battling and it can't be taken very seriously. If you want to start a recreational discussion about Aliens and Predators duking it out, head over to the General Media forum.