I have been for a while debating in my head the relative value of some of the priest talents, especially Mental Agility and Darkness, and I finally decided to establish a baseline computation of how many rating points each of them costs. So hoping this can be of some value to others in making their decision on what talents to drop or not etc.
First of all, Darkness. Three points in there will increase our haste by 3%, multiplicatively. The existence or not of a raidwide 5% buff turns out to not alter the comparison, so I will leave it out.
The main information needed for the computation is that 1 haste rating translates to 0.0078087% haste. In my case, 2551 haste rating translates to 2551*0.0078087=19.92% haste, before talent and buff effects. So my haste with 3 points in Darkness would be: 1.1992*1.03, and no points in Darkness it would be 1.1992.
If we wanted to gain the same benefit as darkness gives, but from haste rating instead, we would need to be obtaining 1.1992*0.03 = 3.6% haste. To obtain that amount of haste, we would need 3.6/0.0078087 = 461 haste rating.
In other words, the 3 points in Darkness are equivalent for me to 461 haste rating. In general if x is your haste from haste rating (0.1992 in my case), then the 3 points in Darkness will correspond to ((1+x) * 0.03 -1)/0.0078087 rating. The more haste you have, the more worth those points in Darkness have.
Now, to estimate Mental Agility's worth. This depends quite a lot on the spec and spell usage, I will here restrict myself to Disc. I think Holy gets even more out of this talent. For Disc, there are 2 main instant cast spells, PW:S and PoM, I will ignore most of the others. Now, PW:S costs 7001 mana without MA points, and I will estimate it being used every 12 seconds. Your numbers will vary here, so you will need to adjust. PoM costs 3706, and I will assume that it also is being used every 12 seconds. Given that MA will save us 10% of the mana costs of these spells, our mp5 savings because of MA would be:
(7001 * 0.1 * 5 /12) + (3706 * 0.1 * 5/12) = 446.125 mp5 savings from all 3 points in MA.
Now the basic formula for spirit regeneration is that S new spirit will give us in-combat 0.5*0.016725*sqrt(Int)*S. With my intellect of 6843 (though should probably use raid-buffed values here), this means that each point of spirit gives 0.6917663 mp5 in combat. In order to reach the extra 446.125 mp5 from MA, we would need:
446.125/0.6917663 = 645 more Spirit needed to match the mp5 savings from MA.
In general, once you estimate the mp5 savings from MA by looking at your instant spells and their usage per 5 seconds, then divide that by 0.5*0.016725*sqrt(Int) (use 0.8 instead of 0.5 if Holy) to find the equivalent points of Spirit in it.
TL;DR:
At 2551 Haste rating, Darkness amounts to 461 rating points.
At 6843 Int with PoM+PW:S every 12 seconds, MA amounts to 645 rating points.
For me this means that point for point, MA is a lot more worth it than Darkness, assuming that I am at a point where I am balancing my secondary stats instead of going all out on one. For instance I have been trying to keep my Spirit around 3100, but running without MA. I will likely start speccing into MA and reforge a lot more spirit out. This also means that if you are comfortable around 2500 Spirit with MA on, you would need to bring your Spirit to 3100 without MA to feel equally comfortable.
I think I'll leave it at that for now, hope someone finds some of this useful/interesting. And some double-checking on my math would be nice, us mathematicians don't do all that well with numbers, surprisingly enough.
If anyone is interested in Holy numbers, and rundown of the frequency of use of your instant spells would be nice. I'm assuming PoM on CD would be a given, and CoH on CD? How often is Renew really used? Other instants (Sanctuary I suppose)?
When comparing a talent that saves mana with a talent that gives throughput, you should keep in mind the length of encounters.
A disc priest with a conventional healing style (shield for rapture and emergencies), is probably gearing for haste > crit > mastery > spirit.
For them darkness will probably be contributing more to throughput then mental agility, as heroic fights in DS are quite short.
A more unconventional disc priest might be spamming more shields and would then probably be gearing for mastery > crit > spirit > haste.
They would probably pick up mental agility.
As for me this is all theory, as I'm still levelling my disc priest, I hope there is some truth in it.
I also researched spell usage by disc priests and put together an attonement spec (31/7/3) that would pickup darkness, while being able to put 2 points in mental agility still, without losing throughput when raid healing and still being a viable tank healer.
The length of the fight is actually not relevant for this question, but the usage of PWS is of course. The point is that once you decide on a frequency of use of PWS, then you can directly compute the mp5 gain of MA. Which means that you can establish exactly how much spirit you can trim off your gear if you pick up MA, or conversely how much extra spirit you would need if you decide to drop it as a talent. I estimated a PWS every 12 seconds, which is a fairly low usage, and already with that MA was way ahead of Darkness.
Just to put it into numbers, myself I was running with 3100 spirit and no points in MA and 3 points in Darkness. That is the amount of spirit I felt comfortable with. What I am now saying is this: If I take the 3 points in MA instead of those in Darkness, then I can drop my spirit to 2500 and feel exactly as comfortable as in my initial setup, because that's how much mp5 the points in MA are buying me. Those 600 points converted to haste rating give me more overall haste, than the 3 points of Darkness were earlier. So by making this change I end up with the exact same mana usage, but with more effective haste. Simply put, MA is more item budget than Darkness, even with a very conservative PWS use of once every 12 seconds.
Now you could argue that maybe you have already reforged all your spirit out and still find you have too much spirit, then Darkness would be winning hands down. But I would then argue it's time to drop some spirit pieces altogether in that case, and still stick with MA.
Where the length of the fight contributes is in how much mp5 you get out of your initial mana pool. This would contribute on what level of spirit you feel comfortable with, and that would be different from fight to fight. But whatever level of mana regen you feel comfortable with, and that would be totally depending on you, you are still better off achieving that level of mana regen through MA, using the resulting freed up item budget in haste.
Well, that makes sense.
Bottom line: when you can't have both talents in your spec, then go for mental agility, as long as you are able to reforge at least 461 spirit to haste for a status quo on througput from haste.
Exactly. Myself I have 3 points in MA and 2 points in Darkness. I had to drop Focused Will to put those points in MA, then reforged a lot of my spirit to get even more haste and other stats. I still need to try to compare Focused Will and Inner Sanctum at some point. A rough calculation showed Inner Sanctum to be about 40-45 mp5, so probably not worth too much, FW will be harder to estimate.
