—The key difference between Armin and Erwin's sacrifice is the way it was portrayed.
While Armin was shown in the light of him giving up his dreams to reach the sea.
Erwin was in the shadow of obsession, of sacrificing the odds to stay alive and reach the basement to satisfy his curiosity and recover that proof for his father, but he sacrificed himself for humanity.
Erwin's goal to reach the basement, which holds the key to secrets of their world that can either tip things in or against their favor against the titans, is considered selfish, but not Armin's simple dream, Sea, which is a childish hope in a world where all have some goal as a driving force behind their actions.
The has point been made, the rest can be evaluated, and hypotheses can ensue.