Armin's dream is to see the sea. While it sounds quite promising and dreamy and hopeful n all, let's face reality. His goal is the sea. And his friends. And his friends are survey corps members. One is the titan; another is ex-survey corp member Annie, who is also a titan and has shown chemistry between them. His best friend is a titan and ackerman who is in love with the said titan, Eren. Can you really tell me that he can command them without hesitation with no bias? He is not confident against Mikasa or Eren. Any talk or fight against their affirmations, & he shuts down.
Yes, Armin has contributed. Be it strategic planning or being manipulative and exploiting Berthold.
But in the first two seasons, Armin flows under the pressure. And when his friends are involved. Be it standing up against shadis for Eren or guilt-tripping Berthold by using Annie for Eren. And during the search, remembering the point of titans in the walls, but Erwin was under custody during that scene, which does not dissolve him of it but still.
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, 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.
That point is made, the rest can be evaluated, and hypotheses can ensue.