As far as I know, Predators cannot breath outside in space. ALIENS CAN!!!! And so, I'll have to agree that Alien will beat Predator... Why? Because Aliens are worse than Cockroaches... they're everywhere you are, that you don't want them to be. And they multiply like crazy, so they're almost impossible to stop.
I don't think they breathe at all, do they? That would explain why they can hang in deep space for a while. I never really put much thought into it. Hypothetically, the derelict spacecraft crashed on Acheron way before the company colonized it, and "made the air more breathable." So there wasn't much of an atmosphere. Which means they can hang in some real crappy environments. Vasquez suggested they nerve gas the whole nest, but she was just speculating it might work. And not having to physically draw in air/exhale means you can remain motionless and undetected for a very long time, like in Aliens when the marines first get ambushed - and all flavors of crap hits the fan.
As far as multiplying, they can't do it without a queen. In theory, if a queen is no longer with a colony, an alien will evolve into one. Who knows how long that would take. This could explain why the alien in
Alien was moving very sluggishly, and slowly when Ripley sees it onboard her escape pod. It may have been trying to become a queen. It knew it was alone, and had to start a colony sometime. They all can't be born queens so conveniently, like Ripley's embryo in the terrible
Aliens 3.
In terms of expendable "nobody" types, the alien has got to be the lowest on the pecking order. And I even think of a typical space marine grunt as being more expendable than a predator is. I'm thinking the predator's on top, seeing how they're far more advanced than we were. If I had to place a bet, I'd go with the predator.