Selenes is DPS increase and dispelling
EoS is high heal and magic shield

The use depends on the group progress and the ability from the sch to heal alone or not

Once you're confident in your healing/dpsing rotation Selenes is a go to, acting like a strong permanent regen
If you're not sure of yourself or if the content you're running is intense/damage and strong magic damage. Go for EoS she will do wonder for your group !

In the end it's only a choice from the sch.