Aliens don't have any particular, shall we say, psychology. They only kill humans to eat and to sustain their hive. While they are extremely aggressive, once they fulfill these needs, for the time being they are satisfied. In other words if you leave them alone they will more or less leave you alone. It's not personal. They have the ability to learn and problem solve, but only so far in pursuit of these aims. They can be outsmarted by people.
Terminators only have one goal which is to exterminate humanity in general or a person in particular that they have been set against. They "live" only to achieve their goal and don't stop until they cannot function anymore. I'd say that makes them the more formidable foe. Although they seem to be defeated rather easily, especially the older models that could be disabled by a shot to the head.