Both raid and mythic+ the top healing source for me is Echo of Light. Echo of light is affected by both crit and vers.
Intellect is basically spellpower.
Spell Power = (Round(StatRating(SpellPower), 0) + TotalIntellect
So for flash heal = 2.03 * [TotalSpellPower] * [TotalHealingMultiplier]
So were does vers come in?
TotalHealingMultiplier = StatMultiplier(HealingMultiplier) * TotalHealingTakenMultiplier * (1 + TotalVersatility)
Total multiplier on healing that you receive. Not that common, but a few tank/healer buffs use it.
Healing Taken Multiplier = StatMultiplier(HealingTakenMultiplier)
Healing Multiplier = 0.05 for holy priest
So vers is a buff to your healing multiplier and increases the amount you heal for each cast by the same amount.
Critical Strike
Total Critical Strike from all sources (gear, buffs, multipliers, temporary procs, etc.) This is your chance to score a critical strike.
All current Critical Strike bonuses are additive.
Crit Healing Multiplier
Total multiplier on critical strike healing. In most cases this multiplier gets applied to the final crit heal (which is 2x normal heal).
So we can see that vers gives a flat buff to healing. Critical strikes whan they happen cause 2x the healing to take place. Both can work together, with mastery.
So vers increase the base heal, then your Crit Healing Multiplier doubles that. The result is taken by mastery and that becomes your echo of light.
Echo of light
Healing = TriggerHeal * MasteryValue(PriestHolyHealing,false) / 2
So, healing * mastery value/2
Duration (sec) = 6 but seen it as 7 ingame
Interval (sec) = 3
Cannot crit.
Thus you can control the amount you get from echo of light by keeping mastery at some level and increasing crit and vers. Crit is more rng but powerful when it happens. Vers is more stable, as it is applied to each time you can a healing spell.
At some point you will reach an optimal balance between all three stats. If you are too powerful, then that extra healing from echo of light is wasted as overheal. Too weak and that means you have to cast more healing spells to make up for it. This hits mana and you could go oom.
Having echo of light as the number one source of healing in your logs is very important.
So ingame I get:
Flash Heal = 3476 From combat log.
Mastery = 38.4%
Versatility = 9.60%
Intellect = 1489
EoL = 667 From combat log
Mathematics
Total healing multiplier = 1.05x1x1.096 (vers) = 1.1508
Flash Heal = 2.03 * 1489 * 1.1508 = 3,478 this is the value of the tool tip for my flash heal. Ingame the flash value will change. Also if there is a crit it will double.
EoL = 3,478 * 0.384 = 667
Crit is 17% or 0.17 which is 1/6. So 1/6 is the probability of a crit. Crit heal means a x2 to the amout healed. The Crit heal Multiplier.
Power Cost = PlayerBaseMana * 0.036 = 50000 * 0.036 = 1,800
Prayer of Healing
Healing = 0.875 * TotalSpellPower * TotalHealingMultiplier = 1,499
Max Targets = 5
1,499 x 5 = 7,496
EoL = 1,499 * 0.384 /2 = 287
287 * 5 = 1,435
Power Cost = PlayerBaseMana * 0.05 = 50000 * 0.05 = 2,500
Crit is 17% or 0.17 which is 1/6. So 1/6 is the probability of a crit. Crit heal means a x2 to the amout healed. The Crit heal Multiplier.
Flash Concentration
0.15 * 3,478 = 3,999.7
EoL = 767.9
total healing = 4,767.6424
Would still take two flash heals to equal or exceed one prayer of healing.
If we go with Harmonious Apparatus, Holy Oration and take the talent Prayer Circle. We can boost AoE even more by reducing CD’s of the holy word spells. Prayer Circle also reduces the cast time of the prayer of healing that follows. Prayer of Mending decreases the CD of the single target holy word Serenity. Circle of Healing reduces the cooldown of Holy Word: Sanctify, along with prayer of healing.
Harmonious Apparatus
Circle of Healing reduces the cooldown of Holy Word: Sanctify, Prayer of Mending reduces the cooldown of Holy Word: Serenity, and Holy Fire reduces the cooldown of Holy Word: Chastise by 4 sec.
Thats with vers which is the raid build. vers and mastery. So with Flash heal you do approx. 4,145 hp of healing total for 1800 mana. For Prayer of Healing you do approx. 8,931 hp of healing total for 2500 mana. It would take 2.15 flash heals to equal a single Prayer of Healing. So three casts or 5,400 mana to match or exceed one cast of Prater of Healing at 2500 mana (you have to cast flash heal three times but two flash heals are close, very close. Still if you take two flash heals its 3500 mana vs 2500 Prayer of Healing wins). This makes flash heal about twice as expensive as Prayer of Healing if all healing counts.
So if a group needs healing you would never spam flash heal. Both from a healing and mana standpoint. This is for mythic+. All players are likely within 40 yards, need healing and there is no other healer. Thus a high chance for all/most the healing from Prayer of Healing being used. Even if there was a crit, it still takes a flash heal crit and a normal flash heal to reach or exceed prayer of healing. 3000 vs 2500 mana.
Sources: ask mr robot and world of warcraft combat log.
Updates to fix maths mistakes. Conclusions changed as needed.