Im going to borrow a post I saw on wowhead about the shamans mastery effect.
Considering in raids, a lot of healing is done on targets at the 60% level or lower, (a lot of the time, near death healing is common) and virtually all is done at lower than 80%, mastery is truly a good stat to have. In addition to this, it increases HPM, while haste does not.
On the other side, crit brings added hps for ELW, ES and Riptide hot, while Mastery effects none of these. So while the breakpoints are rather high, the healing from non mastery sources means crit can hold a higher value point for point. This means that for overall healing, the targets you heal need to be lower than the breakpoints. In addition, crit gives increased mana regen and further intangible benefit from the 10% damage reduction buff.
So where does this all lead? Which should I stack? Both seem good. Well the answer is both are good. But there are generalizations where one might hold more value over the other. As a general rule, if you're finding you're constantly healing people below 50%-60% hp, then mastery will be better for you. If you're constantly just keeping people topped up, then crit will be better for you. From anecdotal information from other shaman and from my own personal results, the best breakdowns are:
For 5man heroics: Stack crit over mastery
For 25man/10man tank healing and farm/easy pug content: Stack crit over mastery
For 25man/10man progression raid healing: Stack mastery over crit
The reasoning is that for 5mans, you're rarely dealing with a lot of people who are low life. Since you're mostly just topping people up with HW and Riptide because of easily maintainable damage, people rarely get low enough hp for mastery to really shine. For tank healing it's obvious that the 10% damage buff is just too good to pass on so crit needs to be a priority. Not to mention you're single target focusing with raid assist. The increased mana gain in the long run from all your single target spells will allow you to bomb big heals longer than mastery will give you.
But for progression raid healing, things change. For starters, a LOT of the encounters follow along a path of completely destroying the raid to near death over and over. Even with crit effecting your hots, the overall benefit from mastery on your HR and CH powers through this and gives your heals a completely hax level of healing done. For these types of encounters where you're constantly healing people at or below that 50-60% threshold, mastery is easily your best stat. So stack it to the moon over crit."
My personal opinion as of right now is that Holy Priests are under powered when compared to these two classes and since Resto Druids are already out classed by Priests I won't be including them even though they are getting buffed soon. I have also seen Disc Priests getting big numbers in total heals through Atonement, Divine Aegis, Spirit Shell, and Prayer of Healing.