It depends on the mthod of combat allowed. Hand to Hand (claw to claw?) an Alien would kick a Predator's butt every time, but if the Predator was allowed to use its arsenal of weapons you'd have to go with the Predator.

I think it should be a fair fight though mano a mano, or beasto a beasto and in that case Predator gets mauled and disolved to death very quickly.