Neither on its own are the greatest examples of realism imo, but when it comes to hammy, action figure archetype, over-the-top exaggerated characters and
cast... Predator wins hands down as it should be. Both the cast and
characters in Aliens are much more grounded in reality than Predator in comparison. Of course, if we compare Predator and Alien, Aliens stands at the halfway mark.
Unfortunately following Predator 2, I've noticed a very slight change in casting trajectory in Predators, which became further amplified in The Predator, a path that has moved away from casting
larger than life protagonist(s), in detrement to the franchise (imo).