Personally I levelled Aegir before I got my Yunan and was very happy with him. In addition to him being my main tank then, I also used him in blue stacks & against red titans galore. His Healing for Damage and damage link was key in keeping me alive against some of the big nasty red titans (11* +)
These days I run Yunan in my tank position wherever I can. Aegir rarely gets a run in defence teams because of this. So yes, you are right that Yunan is the better tank.
HOWEVER, Aegir still has many uses beyond the defence team. Like I said above, he’s pretty good in a blue stack etc…
In terms of your options, I would say to probably leave Aegir at 3-70 for the time being. Work on Issy up to 3-70 too & play with the two of them. Whoever you feel is going to be better for YOU, give the scopes to then
In terms of your defence team I can’t really make a full suggestion but I would say avoid putting both Yunan & Aegir on the same defence. 2 reasons for this:
together they become a really slow defence team & susceptible to tiles
Aegir is passive. The only position I would advocate its OK to be passive is in tank… everywhere else has to either support the tank or hurt the enemy.
Hope this helps
Again, my recommendation is to take both Aegir & Issy to 3-70, use them a bit & decide from there.
If you’re f2p, I’d max aegir over isarnia… she’ll get replaced eventually by either emblemed Grimm or possibly Athena. Aegir’s special is unique and the shared damage plus heal will help you through the map and some of the event stages.