I have both Athena and Ariel maxed, in that particular order. To be fair, I got Athena first and then later on Ariel. They were my first and third maxed ice legemdaries. My second one was Frida.
IMHO, while Ariel is a superb healer by reason that she is average mana and that she cleanses and boosts mana generation, if I were in OP shoes, I’d max first the HOTM. My reasoning is that there is no other hero that deals regular defense debuff to target and nearby up to -65%, except 3* costumed Brienne who debuffs the defense of all enemies. Athena also deals damage and damage-dealing heroes make you win battles quicker and easier, dealing extra damage against red enemies. She is great in all aspects of the game, may it be PVP, PVE, titans, etc.
Besides, there is really no rule that one is required to have a 5* healer. Kiril can serve that function, although he doesn’t cleanse and boosts mana generation, but he buffs attack and defense. Just emblem him to a decent level and he can be as sturdy to serve his functions. 3/70 Ariel is decently usable. I have several 5* healers left at 3/70 in Vivica and Kunchen, but preferred to use Woolerton/Gullinbursti and costumed Rigard/costumed Sabina as healers. That way, I can use those ascension mats to other better legendaries that can serve my playstyle.
But that’s just me.