For a level 70 Priest:
-
1 Intellect = 0.5 non-casting MP5 & 0.15 casting MP5. Ergo, you need 7 int for 1 mp5 (while casting)
-
1 Spirit = 1.0 non-casting MP5 & 0.3 casting MP5. Ergo, you need 4 spirit for 1 mp5 (while casting).
However:
Example:
Flash Heal (Rank 9) base mana cost is 470.
470/15 = 31.333333 (etc.). So, you’d need +32 intellect for the cost of a max rank Flash Heal.
With that 32 Intellect, you’re also getting 4.7 mp5 (32*.15 = 4.7; easy numbers here).
So, in terms of mp5, intellect is half as useful. So it’s a trade-off between higher regen from Spirit and pure mana itself.
STOP HERE IF YOU WANT NO MORE NUMBERS/MATH.
But for a quick further explanation, see below:
Of course, for both int and spirit, there are other benefits from our talents as well. As a disc healer, I get +10% mana via talents. So, +1 int gives me +16.5. Therefore, I only need +28 int to give me one max rank Flash Heal.
Another great example is if you have the talent mental agility (reduces instant cast spell mana cost by 10%). Max rank PW:S base mana is 600, but for me, it’s now 540, which makes it more useful and more mana efficient. If not specced into +10% mana, you’d need +40 int for the cost of one PW:S, whereas being specced into means you’d need +33.
For reference, a Brilliant Dawnstone gives +8 int, so in theory, if not talented into it the +10% mana, I get 9 PW:S casts for every 8 PW:S casts based on mana alone.
Of course, there are also talents that increase bonus healing based on spirit (imp. divine spirit, spiritual healing). Just another factor in your decision. Again, these are just examples and don’t take into account play style, raid composition, which boss fight, etc. Just wanted to throw in a few things to help add in addition to thinking about strict mp5.
Hope this helps!