There is still a debate up to now whether the decision of saving Armin over Erwin is a right decision or not. In Episode 55 of the Series (Season 3 Part 2, Episode 6) Levi has given 2 hard choices of who will he save between Armin and Erwin. And he chose to save Armin at the end. Both of them can be a good commander and tactician because of their sharp minds to lead and make strategies in battle. But their personalities are completely different. So is it a worthy decision to choose Armin?
Armin was hailed as a young genius. He accomplished many things throughout the story (I will enumerate them later). Even Commander Erwin himself praised him for his sharp mind that are very useful in battle. The only problem of Armin is he lacks confidence. He even doubted himself several times but thanks to Eren he can cope with this weakness in a very serious situations.
Meanwhile Erwin possess incredible charisma as a leader. He got steel resolve that no one can break. His judgement are always on point and his subordinates trusted him completely. And the problem about him is, He don't have a will to live. He can easily offer his life for the sake of humanity and bring the burden and guilt of his dead comrades into his grave. I may be admire him as a person but as the character in this series, It's a disappointment. He could've been a key to victory but he chose to die.
Then...
Is it a worthy decision to choose Armin over Erwin? And what could have be the reasons that makes Armin worthy to be saved. Okay so lets enumerate Armin's accomplishments first before deciding if he is worthy.
Battle to Retake Trost District
This epic arc is one of the best arc in Attack on Titan. And for those who didn't know, Armin is the one who make that plan. He propose it to Weilman in order to save and prove that Eren is not the enemy. He cauragely stood up to persuade them that Eren is on humanity's side. And thanks to that they are saved and the plan to plug the hole in the wall using a large boulder went smoothly. I personally think that its a half baked since they don't have assurance that Eren can lift that huge boulder. Well its a success though and it's all thanks to Armin.
Discovering the Identity of Female Titan
The one who discover that Annie is actually the female titan is also Armin. He doubted her it at first when he saw Annie wearing the ODM Gear of Marco Bott. He keep it at himself since he doesn't have any proof. And after the Female Titan incident, he share this information to commander Erwin and devices a plan to prove and capture Annie. And once again, his deduction at this arc is also on point. They defeated Annie and managed to capture her (although they can't interrogate her because of crystallization) but still they succeed and its all because of Armin's sharp mind.
He Found Out Reiner's Hiding Spot (Battle in Shiganshina)
This point, Erwin entrusted Armin to lead the search for Riener and Bertolt. He found out that they must be hiding inside the walls and commanded everyone to search for a hollow part of wall.Thanks to this that they almost defeated the Armored titan and this played a big role towards their victory.
They Defeated the Colossal Titan
Armin makes a decision to risk his life in order to defeat the colossal titan. In this part he mustered up all his courage to device a plan for the defeat of colossal titan. Well his plan succeed and they successfully captured Bertolt but it is also the reason for the hard decision that Levi must choose. If you think really think rationally. They actually defeated the colossal and armored titan but on Erwin and Levi sides failed at taking down Zeke. It can also be a factor that makes Levi change decision.
Its true that Armin's accomplishments in the entire series are commendable. And if I answer the question whether he is worthy or not, I'll gladly say Yes. People might say that its all for the plot and I wont deny that. But still choosing him isn't a wrong choice. He already proved his wort and we know that he knows Eren more, than Erwin is (which they believe that he is the key for humanity's freedom). And besides Levi didn't save Armin. He says that he choose to let Erwin die. He already burdened with so much guilt and responsibility. He also accomplished so much that its unfair for him to let him rest. If you still doubt the worth of Armin, I suggest you to rewatch the whole series and pay close attention to him.
You made a point and throughout the series, after rewatching it, you could understand that the plot was saying something like the young guns has the potential to replace the experts and be one themselves. Just like the one who can sniff titan from a far, we could say that he was good at detection through his nose but he died during the introduction of beast titan. Then who could replace him? It was Sasha who at the same time sensed what he could. Through her instinct, she was able to tell titans are coming from a far. Same with the case of Armin and Erwin :)) and just like Mikasa and Levi. I am really looking forward to Hange's successor if she ever had one.