they both have their own unique strengths & weaknesses. The predator's main weakness is that their body is not designed as a killing instrument, their proportions & attributes are quite similar to a humans, meaning that without weapons, they are quite vulnerable to alien attack, although their brute-strength & combat abilities do somewhat make up for this though. Another weakness which has been exploited by their prey on several occasions, is their tendancy to become over-confident in battle, they relax their guard & allow their prey time to regroup & prepare a counter-offensive, a major tactical weakness.
The aliens also have their share of faults too, their inability to attack targets at a distance is a gargantuan flaw, meaning that any opposition brandishing firearms can quite easily shred them to pieces if they see them first. Secondly, the need for other living organisms to procreate more of their kind is a major weakness, as a chestburster is extremelly vulnerable while gestating inside a host, & countless adults could die in the attempt to gather more hosts.