At lvl 11 mana troop = 9% mana boost. For offense, it will make a difference for Khiona more than Seshat right now (10 tiles → 9 tiles), which will bring Khiona and Seshat needing almost the same amount of tiles (she needs 8 RN even with mana troop + element link mana boost). You’ll want level 17 to go from 8 tiles to 7 tiles for Seshat (considering the mana boost she gives herself too).
For defense, they both benefit: Optimal Mana Bonus Numbers for Every Mana Speed (Now including Ninja Heroes)
Khiona is good in defense, where her berserk boost soaks up tile damage and hits back the attacker hard; on offense it’s only okay, it works well against yellow titans when stacked with Wu Kong/Ranvir or Tarlak (she’s the only dark 5* hero that boosts attack). But with a 3 turn limit it’s got limits otherwise.
Seshat is overall the more flexible hero, being a fast sniper that is naturally the case. So I pick her.