Those are both very tempting options! Armin getting involved in the radio just sounds cool, since he's the type that likes to formulate plans and make speeches. But I think in a people sort of way, Armin might get along better with Septimus. He's very used to taking commands and being ordered around and is comfortable with being a "tool" from being in the Recon Corps. Even if he noticed Septimus treating him that way, he would be fine with it. It would probably be a welcome relief.
Also Armin is 100% happy to help anyone with his translation powers! I've also found it makes a wonderful icebreaker. He used to help out Bianca with the hospital sometimes too, before she left, so he's slightly familiar with Eshai Memorial.
edit way later: I also really like Septimus' username, "survival isn't living" is sort of a theme in AoT, half on Armin's inspiration to fight in canon is to break past the walls and no longer live like "livestock in a pen of humiliation". He narrates the anime and says it like, every episode in the intro lol.
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Also Armin is 100% happy to help anyone with his translation powers! I've also found it makes a wonderful icebreaker. He used to help out Bianca with the hospital sometimes too, before she left, so he's slightly familiar with Eshai Memorial.
edit way later: I also really like Septimus' username, "survival isn't living" is sort of a theme in AoT, half on Armin's inspiration to fight in canon is to break past the walls and no longer live like "livestock in a pen of humiliation". He narrates the anime and says it like, every episode in the intro lol.