The problem here is that different genres are defined in different ways. To give the most obvious examples: musicals are defined by songs, and comedies are defined by jokes. So just because the horror genre is defined by its plot beats does not mean that other genres are defined by the same measure.
Sci-fi has no characteristic plot beats. You can get sci-fi with horror plot beats, you can get sci-fi with romance plot beats (e.g Starman), you can get sci-fi comedies (e.g. Galaxy Quest), you get sci-fi with detective murder mystery plot beats (e.g. I Robot), you get sci-fi courtroom drama (ST:TNG The Measure of a Man), sci-fi police procedural (e.g. Almost Human).
What defines sci-fi is its premise and setting. (Although, one could say that premise is a component of setting: Setting = Time and Place + the initial conditions that allow the plot to take place i.e. the Premise).
So taking that into account, trying to evaluate the relative importance of horror vs sci-fi in a combined sci-fi/horror story is like asking which is most important in defining a car: engine or wheels? They both contribute to the definition of car without competing with each other.
TC