Scholar Fairy, which to use?

[Sylvina]

Both fairies is worthless now for healing. All healing spells heal for more than they do now.

one of them isn't supposed to heal it buffs. I'm asking if its better to use that one instead.

[Cenerae]

one of them isn't supposed to heal it buffs. I'm asking if its better to use that one instead.

Both faries have the same basic healing capabilities. The differences only exist in their cooldowns.

Selene can silence (which is typically irrelevant while also being finnicky to use), dispel in an AoE (which is irrelevant in almost all cases) and can buff 3% attack speed in an AoE with 50% uptime.

Eos has an AoE regen with a 33% uptime, an AoE magic resist buff, and an AoE healing recieved buff.

Eos has far more useful abilities for you, so you'll want to use her most of the time. However, if you want to make use of Selene's buff still, you could try summoning her before a boss fight, using Fey Wind and then Dissipation her away after quickly burning through your initial aetherflow stacks. And then swiftcast Eos back out afterwards.