Emblem for Costume Thalassa and field the costume by default.
The costume gives up only 50 boosted health on the first cast—500 instead of 550—but its fully charged cast gives 800 boosted health instead of 650 and four turns of status-ailment immunity instead of three. Both versions can receive the costume-unlocked insanity resistance, so that is not a reason to prefer one form over the other. ([GitHub][1])
Use regular Thalassa as a situational alternative when the enemy relies heavily on mana reduction or effects that prevent Special Skills. Her Cleric-class Manashield can be more useful in that matchup than the costume’s Sorcerer-class talent. For normal raid, war, and event use, the costume’s stronger second cast is generally more valuable. ([empiresandpuzzles.com][2])