Skadi is situational and does extremely well against minion heroes and better mana speed but not much against titans
Isarnia does well in all situations besides being slow speed especially with her high attack stats and def down. Combo her with weak AOE heroes like Zim and Kadilen is good enough to finish off almost any team.
Since you are looking to do better on titans, I would go for Isarnia. As you are āonlyā fighting 5* you could take her to 3.70 and still get her def down.
If you have C-Magni already then Isarnia is a bit redundant. Yeah she has a better defense down, but I wouldnāt level Isarnia just for titans. Iād go Skadi first given how much she helps in the game in general.
since minion summoners becoming more and more common, Skadi will be very valuable. she is the best anti-minion hero in the game. Only if you have lord loki or finley you can move skadi back to the line