The orthodoxy for holy priests is that, other than spirit, mastery is the best secondary stat to gear for. Haste is also solid (many suggest stacking at least to 12.5% raid-buffed for the first healing-over-time breakpoint--where Renew and Divine Hymn attain an extra tick per cast) and critical strike is shunned. Shunned!
I'm not going to discuss haste in this post, because haste is a bit of a different animal...it's essentially always good for reducing GCDs or cast times and becomes even better if you're approaching a haste breakpoint, but it comes at a cost--to realize most of the throughput gains, you have to fit in more casts, which means more mana burned. Haste is pure throughput at the cost of mana sustainability. (Extra healing-over-time ticks are an exception, but holy priests aren't nearly as HoT-centric as, say, restoration druids.)
What I want to discuss is mastery and critical strike...the two stats that provide throughput at no sustainability cost (i.e. free healing).
What's also interesting about this question is that I'm comparing the two stats that most holy priests would say have a wide gap between them and a clear "right answer" for which is better: mastery. And yet, I'm going to argue the reverse, that critical strike is the correct option for holy priests in most cases.
Front-Loading And Back-Loading
In a vacuum, holy's mastery mechanic--Echo of Light--actually performs quite well. Due to the lack of talents that leverage critical strike (unlike discipline, which has the Divine Aegis mechanic), mastery tends to perform better than critical strike when theory-crafted.
The problem arises when you dig into how Echo of Light works. If you cast a single spell, Echo is rather straight-forward...the target of the healing spell receives a HoT that heals for a certain percentage of the original heal over six seconds.
This gets complicated as you cast more and more spells. If you already have an Echo of Light HoT ticking on a player and heal that player again, more Echo of Light has to be added to the player. Thankfully, the new Echo doesn't clip or overwrite the old Echo, the way the Prayer of Healing glyph HoT does...that would neuter the mechanic terribly. How Blizzard made this work is interesting: it adds up the healing remaining on the old Echo (some of it has already ticked away), combines it with the healing that the new Echo would provide and creates a brand new six second HoT with all of that healing. It's clean and elegant...none of the Echo healing is lost, you just have a bigger and stronger HoT.
The problem, however, is that this serves to increasingly back-load the healing that Echo of Light is providing...that is, it pushes more and more of the healing to the future. With each heal of yours that lands, you're building a stronger and stronger HoT. Chain-casting to get through a rough patch of raid healing with leave you with a super-charged HoT on anyone who received a number of your heals.
What's wrong with that? The problem with that is that the more back-loaded your healing becomes, the more likely it is that it'll get pushed to overhealing by the healing-over-time from other healers or the completions of casts in progress. Now you might fairly point out that if it weren't back-loaded, then you'd be pushing other healers' healing into overhealing and, assuming we don't care about meters performance (and we shouldn't), what's the difference?
That response would be right on point if we assumed that the same casts would be made in either case. However, healers adjust their spell choice based on the state of life bars. If all that Echo of Light healing were delivered right up front, instantly, the other healers would see the effect of it before they made choices to drop another Healing Rain or cast a Holy Radiance or begin another Prayer of Healing cast.
In other words, if the effects of your Echo changed life bars quickly, it might save your fellow healers some casts, thereby saving them some mana to be used at another time. Less total overhealing, rather than overhealing redistributed. Instead, since your Echo's real effect is still to come, they cast all those spells, largely topping everyone off and then your Echo roars in to overheal all those people.
Now, what if we had a similar mechanic, whereby each of our heals added extra healing...but that healing was reflected instantly in life bars, thereby allowing your fellow healers to adjust their casting decisions accordingly? Wouldn't that be nice?
It would, and that mechanic is called critical strike. Now, you may have heard some scurrilous rumors about critical strike, that it's unreliable and RNG...but those fears are largely overblown. And while I shan't rehash my entire post on that subject, I will re-affirm that critical strike is quite a different animal for a holy priest, especially a holy priest who is raid healing.
A raid healing holy priest is going to make heavy use of three spells primarily: Prayer of Healing, Circle of Healing and Holy Word: Sanctuary. She may or may not also make heavy use of Renew. (This is not to say such a priest would not use other spells, but those spells would do the majority of the heavy lifting.)
The common thread between all of those spells is lots of "hits" per cast. Prayer of Healing hits five targets per cast. Circle of Healing hits five targets per cast (six if glyphed). Renew ticks four times, or five times if you're at the quite attainable haste breakpoint of 12.5%. Holy Word: Sanctuary is variable, since it depends on how many people are standing in it, but assuming you choose to use it when a lot of people can be inside it, it hits a lot of people and it ticks nine times (base, without the four-piece tier 13 bonus).
When you have that many hits per cast, you're going to get a lot of critical strikes. Unlike single-target healers, where you're likely to get nothing on any individual cast, with these sorts of spells, you're quite likely to get at least one critical strike per cast, if not more. This, in effect, acts as a similar effect to mastery (I'm not speaking mathematically, here, but rather conceptually); each of your casts in effect gets some extra healing. Not RNG healing, mind you, but a pretty predictable return since each cast has enough chances for "mini-crits" that you are very close to guaranteed to get some.
The difference, of course, is that this return will be front-loaded. It happens as soon as you make the cast, or as soon as the tick happens (in the case of Renew or Holy Word: Sanctuary). You see the healing immediately and so do your fellow healers. It won't be overhealing for you (assuming you cast when there was damage to be healed) and it hopefully won't lead to overhealing for your other healers if they see and adjust.
Now, can Echo of Light still do yeoman's work? Absolutely...in fights where your healing team is really struggling to stay abreast of damage the entire fight, and therefore everyone is not eventually topped off, then your Echo of Light will be able to tick fully and mastery does have a better conversion rate from rating to throughput gain. In cases where you are never really topping off your raid, mastery is a stronger stat.
However, fights like that are very rare unless you're trying fights that you don't really have the throughput for. In most fights, you either have periods of strenuous healing that eventually end, allowing you to get the raid topped back off for the next such period, or else you're casting continuously in order to keep the raid largely topped off to be prepared for large damage spikes. In neither scenario is back-loaded healing a big asset, really. Obviously, it's not all wasted...some of those Echo of Light ticks occur while you're casting the next spell, which presumably is still before everyone is topped off to full health. However, the larger and larger ticks keep getting pushed further into the future by each successive cast.
You won't do poorly with a lot of mastery, but you may find that you squeeze more useful healing out of critical strike.
Could this problem with mastery be remedied? I think a fairly simple "fix" would be to make Echo of Light operate the way Wild Growth does on each target it hits: front-load the healing so that the HoT starts off strong and then weakens over its duration and, otherwise, let Echos of Light combine the way that they already do. I believe that that would help balance out the continuous pushing back of stronger HoTs. You'd still be creating stronger HoTs with each successive cast, but it would deliver a big percentage of its payload quickly...which, again, has the virtue of being reflected quickly in the life bars, allowing your healing team to adjust their casting around it.