By Sci Fi species, I'm referring to sentient and sapient species, for example the major races in Star Trek and Star Wars (Vulkans, Klingons, Mon Calamari, Wookies, etc)
Do you think there is any significant difference between the two? Do (or would) you tend to treat them differently in how you play or GM a campaign? Are there implications of being a fantasy race that are not true of a sapient alien race or vice-versa?
Or are they identical concepts in your mind?
Sci-Fi species have an origin founded in science. Fantasy races do not.
Races in D&D were created by gods, primordials, devils, demons, or even formed from the dreams of powerful beings that escaped their minds.
The origins are completely different, and that's not the only place where the differences begin or end. In D&D a place where many people bring their prejudices and it fails them is in the concepts of good, evil, law, and order. In D&D these concepts are building blocks of the universe. In science fiction they're ideas to explore or comment on. In D&D, there are creatures who are the living embodiment of dark emotions like the Sorrowsworn. In D&D, darkness is a force, and not merely the absence of light. Races like Shardar Kai are the Elves of Shadowfell, who changed from both their environment and worshipping a non-Elven diety. Because this is true in D&D, there are also races like the Yuan-Ti who were once human. They turned into snakes not because of magic spells, evolution, or potions but because they devoted themselves to an evil snake god. The act of worship in Sci-Fi will not turn your entire race into snakes. The list goes on and on.
So I'd say the real differences are in the framework of the genres. Sci-fi, at a minimum, has the veneer of science. Fantasy sets its own rules for the universe and then fits creatures, creations, and worlds into it, or visa versa. Fantasy is not bound by natural laws found outside of its own pages.