1. What's all this nonsense about Haste vs. Mastery
Let's start out with what Haste does for you:
- Increases your spell casting speed
- Decreases your global cooldown (GCD)
"But if I cast faster doesn't that mean I cast more spells and thus I will go out of mana faster?!" - Priest
Yes and no. Haste is extremely beneficial in many, many situations. The #1 benefit of Haste is
saving people's lives. Yes, 0.1 second can really mean the difference between life and death. I'm sure you've all been in that situation of "my heal was a fraction of a second from going off!" when discussing why a tank/player perished.
Also, generally raid damage comes in waves: burst -> lull -> burst. Therefore you are not spam casting non-stop (at least in most situations) and thus you are getting pure gain from being able to get your heals off faster and everyone stabilized before the next burst.
Haste is considered to provide purely an HPS (heal per second) increase.
Now let's shift our focus to what Mastery does for you:
- Your direct healing spells heal for an additional percent over 6 sec. Each point of Mastery provides an additional 1.25% of healing over 6 sec.
So what does this mean? It means that when you cast Prayer of Healing on a group and you are hitting all 5 players in the group you are healing them for: direct PoH heal, Glyph of PoH (assuming you have it, which you should!), and then another HoT, Echo of Light.
Mastery proves to be useful in instances where the entire raid takes massive amounts of damage and your EoL will have the ability to fully tick 6 sec. with little to no overheal. It's also very important to keep in mind that the more Mastery you have, the bigger each tick from EoL becomes. So even if you are consistently seeing 30% overheal on your EoL as your Mastery increases, this is okay. By each tick hitting for more you are essentially saving more lives because you are filling up that HP bar much faster.
Mastery provides an increase in HPM (heal per mana) and HPS.
"Okay I understand all of that but how does it apply to me? Which stat should I choose? Should I mix them together or just stack one?" - Priest
There's no right or wrong answer here, unfortunately. I wish it was as bread and butter as some other classes/specs, but it's not. I will, however, provide some guidelines to help you make the best choice for certain thresholds.