What would be the most difficult thing to fight, an alien, or a predator?

I was watching Predator today and I was thinking, man that would suck if one of those was after me. But I remember Alien from a while back, and having one of them after me would be just as bad. Or so it seems…one can regenerate most wounds and spits acid, the other is tougher than a tank and has all these gadgets that would frighten God.
Say you’re up against one of them and you have limited resources, or at least any of the equipment that was featured in both the first movies for each, (Switch them around if you want.) which one of these do you think might be the harder challenge?