Since there are so many new holy priests in pvp these days, unsure about how they should gear, I decided to explain everything you need to know about crit, haste and mastery.
I will start with comparing crit and mastery, and to do that, I collect statistics in Recount from five ordinary 3v3 matches. The outcome looks like this (you can do this yourself if you really want to know the optimal stat for you, or if you believe you have a very different play style) http://imgur.com/ewal7xW http://imgur.com/ryp5CGl
Some info we will need:
Mastery gives 0.0021 percent per point. Crit gives 0.0016 percent per point.
6000 mastery = 12.6% bonus healing. 6000 crit = 9.6% bonus healing.
Crit will boost everything except PW: Shield and Rapid renewal, while mastery will boost everything except PW: Sheild, Renew and Lightwell.
Crit will boost 76,9% of my healing by 9.6%
Mastery will boost 56,6% of my healing by 12.6%
76.9+9.6%+1.5(rapid renewal unboosted)=85.78
56.6+12.6%+12.8+9(renew and lightwell unboosted)=85,53
As you can see, crit or mastery will give an almost identical result in your healing. Some times, Blizzard actually knows how to balance things.
What you should choose depends on your comp.
The bonus healing from a crit is much more likely to be wasted as overhealing compared to the mastery HoT.
Most players want to rely on RNG as little as possible, as well as prefering a less "jumpy" health bar.
Better when playing with a class such as mage, who tend to recieve a big chunk of damage followed by a moment with no damage taken. Classes like this prefer stronger direct heals over HoTs.
Now what does haste do for us?
Haste, as you most likely already know, has three effects:
1. shorter cast times
2. shorter GCD
3. extra HoT ticks
Effect number 1 is almost completely wasted since a holy almost never casts anything. Lightwell and Mass dispel is in no need for a shorter cast time. Flash Heal is only used if you are undergeared or are using a suboptimal rotation. The hymns are the only spells making use of a shorter cast time, and you will most likely be casting them from a safe place anyways.
If you sacrifice your crit/mastery for effect number 2, a shorter GCD, you will complete your healing rotation of instant heals faster, but it will heal for less, increasing your chance to get into the situation where all your heals are on cooldown but your partners still need healing,
forcing you into using the inferior heals like flash/greater/binding heal. And since those are cast time heals, chances are that haste might actually make your healing slower.
Additionally, the more haste you have, the harder it will be to refresh Renew with Serenity, costing you both mana and GCDs.
About effect number 3, lets take 6000 haste like we did with crit and mastery. This will give you one extra tick on Renew and Lightspring, which in this case equals to about 3,5% bonus healing, a lot less than crit/mastery would provide you with.
Mastery is best when playing with classes which recieve an even ammount of damage throughout the game. Classes who rely on high armor and damage reduction abilities for survival. Example: Warrior, DK, Shaman, Druid
Crit is best when playing with classes which recieve an uneven ammount of damage throughout the game. Classes who rely on movement and big immunity cooldowns for survival. Example: Mage, Warlock, Hunter, Rogue
Haste will give you an edge when dispeling, hymning and smiting, all secondary things that you will spend a small ammount of time in your games doing. I would still recommend haste until you are fully geared, since you will have to cast more flash/greater heals the lower your gear is. When fully geared, and familiar with the spec, I would recommend swapping most of the haste into mastery or crit (depending on comp you plan to play) to boost your instants and move away from the casting style of gameplay, and make full use of holy's greatest strength.