I think the two should be depicted as roughly equal in strength, but the nature of their strength should be different.
With its long slender limbs and fingers, the Alien's advantage in a fight would be its leverage. The Predator would rely more on pure muscle.
The Alien's strength would be steady and unyielding. The Predator may be capable of bursts that could momentarily overpower it, but if he doesn't finish the fight quickly, he'll be in trouble.
It needs to be said, of course, that an Alien's natural weapons should mean that a Predator would never want to grapple with it. If things comes to that, the Predator has already been beaten at his own game and become the hunted.