Powerful Void (Mana Leech) is almost mandatory because of all the benefits it brings, especially for Druids. So the question that remains is whether you should use Strike (Critical Hit) or Epiphany (Magic Level bonus). In my opinion, it depends on a few factors.
The difference between the two is that Strike will only increase your damage, whereas Epiphany will increase both your damage and your healing. It's hard to estimate but it's generally accepted that the damage increase of Epiphany is lower than the that of Strike.
So, you have to ask yourself what do you want more, damage or healing power?
When hunting solo, do you feel like your Healing Spells are not healing you to full HP? This will start happening to mages around level 600 or so depending on how low your health gets before healing. If that seems like a problem and you can't improve it further with other changes (different equipment or imbuements on other parts of your set), then perhaps Epiphany will increase your survivability. Otherwise, Strike is probably the way to go, as more damage means higher exp/h and loot/h.
The same logic applies to duo or team hunts, but now you also have to consider your proportional contributions to the party. If your knight is higher level/skilled or has the vocation advantage in the spawn and does a lot more damage than you, or if simply the damage he/she takes is frequently higher than your healing, then you may want to use Epiphany. On the other hand, if you're the highest damage dealer and healing is not a problem in the hunts you go, use Strike to increase your firepower even more.
TL;DR
- On one slot, always use Void because it helps you hunt longer and helps with micro-managing your mana pool
- On the other slot:
- Use Epiphany as a more defensive/balanced option to increase both your healing and damage
- Use Strike as an offensive option to focus on increasing your damage as much as you can