From the usefulness of the ER4 on raids, definitely druid first.
Druids now have great heals and often end up helping out in CH chains on raids, but the good druid heals are 60+. Those require ER4 to get the range. And the range is a must for all healers now on raids.
A cleric's CH is level 49, ER3 (supplescale armband) and even ER2 (moonstone ring) is sufficient and will work for CH for the range. Supernal Remedy has 2x the range of CH, so their 60+ heals aren't an issue for needing ER.
It's definitely a great item for both, but would equip druids first because their heals require the ER4, cleric heals do not.