Originally Posted by
DarkwingDuck
Not that this is to turn into yet another discussion on the topic, but I felt like I wanted to point out a few things:
Mastery directly affects your entire selection of spells, while Haste only affects a few spells.
While it is somewhat playstyle dependant, be aware that the strength of Resto Druids has shifted almost completely from MoP to WoD. Previously we were very good at keeping the raid stable with high, sustained, and mana-efficient healing per second. This is no longer the case. Our main strength now lies in our ability to deal with high amounts of raid damage over a short period of time, such as Quake on Twin Orgron or Tectonic Upheaval on Tectus. Tranquility is especially powerful in this regard and it is not affected by Haste in any meaningful way. This is why as of right now, Mastery tends to be slightly stronger than Haste because it enhances our strenghts more than Haste does.