Firstly, we do NOT know weather an Adult Alien or Predator is stronger. We have never seen them go side by side at a powerlifting meet.
Hand to hand, the marine, even a UFC fighter, while tough by human standards, is still just a human. You could take a huge 6'7" 330 pounds of muscle human, a WSM competitor/winner like Magnus Samuelsson, train him with the best fighter UFC coaches on earth for 5 years straight, and he would still get killed by a Lion in combat unarmed and hand to hand. Even the strongest and toughest humans, are still human.
Both the Predator and Alien have been shown to be exceptionally strong. The aliens have been observed doing such feats as punching through solid steel, and the Alien has enough strength to lift a Pred with its tail.
The Predators can lift a human off the ground like he weighs nothing, and jump 25 feet vertically from a static position.
Predators are about 7'6" and 400 pounds on average, and all muscle too. They would also be, most likely, comparatively much stronger and more agile than would a human of equal size and build.
The Alien is hands down more flexible and agile.
The Alien has much harder skin.
The Alien has acid blood.
The Predator "may" have a slight strength advantage.
Completely unarmed, I would say the Alien would win at least 80% of the time. It just has too many natural weapons to fight bare handed and have a high success rate.
With hand to hand weapons available, but no projectile weapons, I would put the advantage to a skilled hunter at least 80% of the time. With a blade a skilled hunter can avoid acid splash decently, and can cut an alien with enough speed to kill it in one attack.
There is no Proof that an Elder Predator is physically stronger than a younger Pred, more experienced with a melee weapon for sure, but still at a disadvantage completely unarmed.
I would not say that the Pred is guaranteed to loose, just it only has about a 20% chance of winning with its bare hands.
Now all of this is also dependant on the type of alien, or rather the host type. All of the above is assuming it is a normal adult alien, with a human host.
We have not yet seen for sure what a Pred-Alien will be like. We know it is still an Alien (or damn well should be), and it may be as strong physically to a Predator as a human hosted Alien is proportionately to us.
If that is the case a Pred-Alien (Alien who had a Predator for a host, not some mutant) would not be an option in barehanded combat any more than an Alien born of us is for us to take on barehanded.
