So tired of Mastery vs Haste post
OP if you go all out on haste or you go all out on Mastery you're doing it wrong,
Mastery is a very nice stat for raid healing, in heavy AE dmg it will be number 2 or 3 on your top heals
The only thing haste is going to do for you healing wise is provide throughput, if you can't get by with 0 Haste you're raid/you are doing something wrong. If you're running OOM you're doing it wrong (or your raid is and haste) If you're ovearhealing by more than 10% you're doing it wrong.
This is not to say you shouldn't pick up haste but after darkness and 5% from raid buff you don't need a whole lot more.
TLDR balance your stats the greatest gain you are going to get is by experience and using the right heal at the right time.