This is just how I interpret it based on what I know.
In Chapter 72, when Levi and Erwin are talking about the plan to defeat the Beast Titan,
it was revealed that, to quote from the wiki,
He cared more about discovering the secrets hidden in Eren's basement, and finally proving his father's theories correct, than he did about retaking the lost territory of Wall Maria. His dream of understanding the world was so strong that he confessed to Levi that he considered it more important than the victory of humanity.
Levi was reminded of Kenny Ackerman who, while dying, told him that everybody has to be drunk with something... This is when Levi realizes that what Kenny said applies to Erwin. Erwin was drunk on curiosity to understand the world than to save the people within the walls. If he would choose Erwin, Erwin would prioritize his dreams rather than their goal of saving humanity.
And then he later says he chose Armin because they pushed Erwin to be a demon. Had Erwin's circumstances been different, had his father not been killed and if he was able to pursue his goals of understanding the world without having to worry about being arrested, he would probably have been a researcher or someone other than a soldier. Levi understands that Erwin only wanted to answer the questions he had about the world. It was never really his desire to become a soldier or be handed the position of someone who should save humanity. If he chose Erwin, sure, Erwin's questions would be answered but it would likely doom humanity as Erwin's goal stops at answering his questions and proving his father right. In an episode of the latest season, I seem to recall Levi asking Erwin what happens after they find the truth in the basement, to which Erwin replied 'I don't know'.
Erwin sacrificed many lives for his ambition whereas Armin sacrificed his life with the intent of Eren achieving their ambitions. Armin was willing to give up his life and prioritize humanity's survival so long as Eren, the one he promised to see the sea with, can achieve their dream for the both of them. Erwin, however, was willing to give up the lives of his subordinates in order to achieve his dream, which only is about answering his questions and proving his father right.