Magni seems very straightforward as a sniper with a defense boost. And he would be fairly useful since my hardest hitting blue 5* are Isarnia and Glenda.
Misandra on the other hand does not hit as hard. However her potential mana boost seems like it could be useful, particularly with my average heroes like Glenda and Ariel.
I have both maxed and very similar blue roster to yours (Ariel, Vela, Aegir).
My most commonly used blue stack is Ariel - Magni - Vela - Frida - Misandra.
When you are looking for damage dealer, and feels to me that with Glenda you are, go for Magni, no doubt about that. He provides the hit that you are lacking. I like Misandra for the mana support she provides - often my fast heroes are charged but my average heroes are missing just a bit of mana - Misandra’s hit can help provide that and that extra mana for Ariel’s heal may mean a won raid. But unfortunately Misandra’s % hit is too low to consider her a sniper, and most of the times she will only hit once when you really need that chain. I find her a decent supporting hero but totally replacable if I pull something else. Magni, on the other hand, is that core sniper every squad should have. My vote goes for him.
Not that this makes her generally useful but I’ve been loving Misandra for rush attack - as someone who only has one set of mana troops above level 11 - I put them on Misandra in a blue stack and suddenly my entire blue team charges in 6 tiles. Fire her and even if she only hits once, that makes up for that half tile difference.
Seems like you have a lot of average speed blue heroes maxed currently and Misandra is great as a support hero in a blue stack to help her slower teammates charge - albeit not as reliably when it’s not rush.
I had a similar debate a few weeks ago too and originally I was thinking magni because he’s predictable and deals more damage. But I changed my mind after playing around with both at 3.70. Yes Misandra hits like a wet noodle but boy is she fun. I don’t have ariel but if you have high level mana troops to put on ariel, misandra may be the ticket to turn around raids for you if she can charge ariel.
I have costume kiril and what convinced me on her was a few times her one hit is what made the difference to charge up kiril right in time on raids - not sure how that math worked exactly tbh bc I had costume bonus, mana troops and mana talent on kiril but apparently there were a few times he was a half tile short of charging and misandra made all the difference. I’m more of a person to level the hero that I find more fun though and not necessarily the most practical. If you haven’t done so already it may be worth bringing both to 3.70 and seeing which one fits your play style more. (Bearing in mind the limitations of reduced stats of course)