1. Introduction
There is no way to properly evaluate each of the stats available to priests in their gear without assuming a certain scenario.

Int is a fixed contribution.
Mana regen depends on the duration of the combat.
Spirit depends on the duration and how often you are on the 5s rule.
Healing power depends on how much you cast what you cast.

So in order to have a decent comparison between these stats you need to come up with a scenario that most likely fills what could be the hardest bosses to heal thru.


2. Math (my lame attempt at)
I did some mathwork some months ago assuming:

Spell cast: flash heal
Duration of the combat: 10 minutes
Time on 5s rule: 50%

[Intelect]
10 int = 165 mana

[Spirit]
Spirit regen formula = 13 + (spi/4)
Variations on Spirit formula = spi/4

100 Spirit = 100 mana per tick/4
25 mana per tick

300 ticks x 25 mana per tick = 7500 mana

100 Spirit = 7500 mana (if out of 5sec rule = 100%)
Standard regen is 50%
Meditation 15% (in 5sec rule)
Transcendence set bonus 15% (in 5sec rule)

0.5(1) [out 5sec rule] + 0,5(0,3) [out 5sec rule] = 0,65 [standard regen]

0,65 x 7500 mana = 4875 mana

100 Spirit = 4875 mana (normal combat scenario)
1 Spirit = 48,75 mana

[Mana Regen]
1 mana regen per 5s tick x 120 ticks = 120 mana

1 mana regen = 120 mana

[Healing Power]
500 Healing Power -> Flash Heal rank 5 -> Healing Efficiency = 3,22 HP/mana (heal i would possibly use on most scenarios)
minus
0 Healing Power -> Flash Heal rank 7 -> Healing Efficiency = 2,6 HP/mana (stardand heal for priests with no healing power)
equals
(Variation in) Healing Power = Y = 0,62 HP/mana

Mana Pool available in standard combat scenario
Int = (372+31)1,1(kings) = 443
Spi = (313+35)1,1(kings) = 382
Mana regen = 63 + 30(wisdom) = 93

Mana(int) = 7310
Mana(spi) = 18593
Mana(MR) = 11760
Mana(total) = 37663

Mana(total) x Y (see above) = 37663 mana x 0,62 HP/mana
37663 mana x 0,62 HP/mana = 23351 HP

23351 HP / 0 Healing Power Mana Efficiency = 23351 HP / 2,6 HP/mana
23351 HP / 2,6 HP/mana =~ 9000 mana

500 Healing Power = 9000 mana
1 Healing Power = 18 mana

[Comparison]
1 int = 16,5 mana
1 spi = 48,75 mana
1 mana regen = 120 mana
1 healing power = 18 mana

1 spi = 2,95 int
1 mana regen = 7,2 int
1 healing power =~ 1 int (due to overhealing)

I assumed Spiritual Healing
I assumed Mental Strenght.

3. Notes

Healing Power: The better your gear (ammount of mana available), the most effective it is. My calculation pushes it to its max efficiency, so it can perform worse in most cases.

4. Priests in 1.10

[Spiritual Guidance]

Yelds a 9.2% increase in spirit mana output following the math given above.

Spiritual Guidance: 25% spirit = +healing
+100 Spirit = +25 healing spells

With talents:
100 Spirit = 4875 mana
25 Healing Spells = 450 mana

100 Spirit = 5325 mana
1 Spirit = 53,25 mana

[Mental Strenght]
Most builds will lack this talent on 1.10 (i think). This should overall reduce the mana contribution given by int by an even larger margin.

5. Desired Feedback

A - I'm closing in on my 8 Trans set bonus. I would like some input on how useful it is on a real raid scenario. It seems godlike on paper. But with a full raid casting GHs, it could be overwritten so often that it never manages to tick. How important do you guys think it is going to be? If it is not overwritten, it could perform better. +healing would work wonders with it, and that should be accounted for.

B - Is this math blatantly wrong on any aspect? Do you disagree on something regarding the math, scenario used, overall thinking? Post your reasons.

C - What are the ratios used by you when choosing gear? How did you come up with that math? What scenario you used?

6. Apologies

I am not an american, as you can probably notice by my poor english. I also didn't check spelling, so some words with double letters could be wrong. My apologies for any hard time reading thru this.

10 mp5 = 4 mana per tick (not 2) = 16 spirit while not casting (=53.33 spirit while casting, assuming 3trans and meditation)

Just to nitpick, Mhedic :)

This mod answers all of your questions about spirit vs. mana/5: http://www.curse-gaming.com/mod.php?addid=2337

Personally depending on where I am and how lazy I'm feeling 1 mana/5 = somewhere between 2 and 3 spirit. Feel free to judge your items accordingly. For example, this means that from a pure regen standpoint Gloves of Prophecy are STILL the best gloves in the game.

Mana regen vs. +healing is a bit more difficult to quantify... the original post gives exactly one situation, and I'm not sure whether his methods are necessarily the way you want to compare them.

I'm not sure the theorycraft can get you very far in terms of comparing mana/mana regen to +healing. You can compute your expected total healing per manabar (extended for fight duration), but would it be very informative?

The comparison between Spi/Int/MP5 is easily worked out, on the other hand, with help of of some emprical data on how much time you spend casting. I have a post on this buried somewhere in our guild forums, but for Priest with Mediation and 5pc Trans, you need about 50% time out of casting to make Spi better than MP5 (throw Kings into the mix, and that number goes down a good bit).

As a Mage, I have a slightly different view on this. I have two gear sets, one is non-set gear with maximum +damage (+631 damage and about 5600 mana unbuffed); the other focuses on MP5 and other stats (about +490 damage and 6500 mana unbuffed). There are two kinds of fights--those where I run out of mana, and those where I don't, and that determines what I wear.

I didn't provide some info that was on my notepad file.

Scenario: 10 min (600 seconds) boss fight
Spirit regen ticks: 300
Mana regen ticks: 120

The mana regen "ticks" are actually 5 second ones. Not the regular 2s ones. The name is not proper, but the math is.

Concerning time expent on 5s rule
I had a mod for some months that displayed how much mana i expent during the fight, how much i regenerated, int/spi ratio, time in 5s rule and ammount healed.

I collected the data for some time and came up with the 50% on most boss normal boss fights. The ratios varied according to the combat, but made sure that spirit is way superior to int in almost all regards, except in extremely quick fights, but those can be ignored in raid scenarios.

I think everyone here knew int stinks in raids already ;)

Oh, whoops, I was thinking about this earlier and mixed up my thoughts while typing this out

edit: I just grab healpoints and assume it knows better than I do when selecting loot... time to get a 2nd mind tap, bye bye seal. your FR won't help in AQ

that sums up my thoughts aswell. get some of each but since the trans set is pretty loaded with +healing my idea is to go mostly for mp5/spirit on the other slots.

well my gear is a complete mish mash at the moment. We finally opened Ahn'Qiraj on Ragnaros-EU yesterday, and i had to wear a flarecore robe because its the only thing i've got :) Granted, the 35 stamina affords is nice, and my overal gear gives me 7.8k mana buffed and 4.5k health. But then the rest of my gear is a mix of Zandalar and Prophecy, with a Blue Dragon and Hero Charm thrown in and some Gloves of Spell Mastery (they were a novelty at the time). I'm currently trying to get a mix of int and mana regen gear, but i always try and extremify my gear (eg Shimmering Geta/Shard of the Scale to cover a lot of mana regen gear in a few slots, once i can get those).

In terms of +healing and mana regen gear, there are some obscenely nice pieces in AQ 40. I doubt i need to mention the Blessed Quiraji Acolyte's Staff as an upgrade to Benediction, because not only does it give a huge + healing boost before the 55 enchant, but it also hase 15 m5. granted, Arlokk's Judgement is still the best spirit and therefore mana regen staff in the game, and i guess is still the weapon combination of choice for mana regen in a fight.

Of course in an ideal world we'd all have Fang of Korialstrasz with a high + healing off hand :)

I prefer a very heavy mp5 build. My reasons are fairly basic. It simplifies the healing process. I dont have to keep track of as many spell ranks. I have the lowest viable rank that my +heal makes useful and then the top ranks. Second, its flexible. If I need to do something other than heal its not going to be a cloudy day for me and I dont need a 2nd set of gear. Lastly, spirit builds depend on Trans 3-Piece too much. I already have gear that is an upgrade and I dont want to be tied to 3 pieces of gear forever with no forseeable upgrade.

1.10 I believe will work well with a regen build as well. With the 2.5 second casting time lower ranks of Heal seem very viable. When combining 130+ mp5, 500 +heal, and sub 250 mana 800-1100 hp Heal R3 & 2; the priest can maintain a very high rate of longevity and a competitive hp/sec. That is not to say I do not think there will be correspondingly good +heal builds, I just have my way. I favor simplicity and versatility.

The hidden benefit I find with mp5 gear is that it also tends to have a significant amount of stamina, and honestly, more hp is never bad.

I value (I have a 60 priest) spirit / concentration very highly, but I will tell you this : 8/8 trans is GOD.

God I tell you.