On fights like the emperors your FSR percentage would be very high, as spike damage is abundant and you're generally always going to be casting something every 5 seconds, I'd go with a 90ish% FSR percentage for these types of fights. However, much more relaxed, stable fights such as razorgore, the prophet, and C'Thun. You're not constantly casting, so naturally you'd have a much lower FSR percentage. Probably around 40-50%ish, maybe even lower. So what does this all mean? Here comes the data.
0% regeneration while casting (NO TRANS BONUS, NO MEDITATION TALENT)
mp5..........spirit........ Time spent in FSR
1.00..........160.00.......99.00%
1.00......... 16.00........ 90.00%
1.00......... 8.00......... 80.00%
1.00......... 5.33......... 70.00%
1.00......... 4.00......... 60.00%
1.00......... 3.20......... 50.00%
1.00......... 2.67......... 40.00%
1.00......... 2.39......... 33.00%
1.00......... 2.29......... 30.00%
1.00......... 2.00......... 20.00%
1.00......... 1.78......... 10.00%
1.00......... 1.60......... 0.00%
-On a fight like the emperors, 16 spirit would be the equivalent of 1 mp/5.
-On a fight like C'Thun, 3.2 Spirit would be the equivalent of 1 mp/5
15% regeneration while casting (meditation OR 3-piece transcendence, IF YOU HAVE BOTH, SEE BELOW (YOU BETTER HAVE BOTH))
mp5..........spirit.........Time spent in FSR[/u]
1.00......... 10.67........100.00%
1.00......... 6.81......... 90.00%
1.00......... 5.00......... 80.00%
1.00......... 3.95......... 70.00%
1.00......... 3.27......... 60.00%
1.00......... 2.78......... 50.00%
1.00......... 2.42......... 40.00%
1.00......... 2.39......... 39.00%
1.00......... 2.15......... 30.00%
1.00......... 1.93......... 20.00%
1.00......... 1.75......... 10.00%
1.00......... 1.60......... 0.01%
-On a fight like the emperors, 6.8 spirit would be the equivalent of 1 mp/5.
-On a fight like C'Thun, 2.4 Spirit would be the equivalent of 1 mp/5
30% regeneration while casting (transcendence and meditation (WHAT EVERY RAIDING PRIEST SHOULD HAVE)
mp5..........spirit......... Time spent in FSR
1.00......... 5.33........ 100.00%
1.00......... 4.32......... 90.00%
1.00......... 3.64......... 80.00%
1.00......... 3.14......... 70.00%
1.00......... 2.76......... 60.00%
1.00......... 2.46......... 50.00%
1.00......... 2.39......... 47.30%
1.00......... 2.22......... 40.00%
1.00......... 2.03......... 30.00%
1.00......... 1.86......... 20.00%
1.00......... 1.72......... 10.00%
1.00......... 1.60......... 0.01%
-On a fight like the emperors, 4.3 spirit would be the equivalent of 1 mp/5.
-On a fight like C'Thun, 2.2 Spirit would be the equivalent of 1 mp/5
So what's the better choice? Depends. Mp/5 Will always ignore the FSR, and the FSR aint' goin no where. However, lower FSR percentage fights will prove to favor spirit, but sadly theres just not enough of these to make a stacked spirit build the winner here. Also, we've never been a guild that's used a healing rotation, meaning all of our priests will most likely have higher FSR percentages. Mp/5 will almost always rule supreme, although some other nice perks from spirit include Innervate and the spirit/healing ratio talent with the dawn of 1.10. Now, I'm by no means saying you should be a regen whore like me, it's the healing whores and the balance whores that round out our group of priests and compliment my playstyle.
Eight piece transcendence is a nice set bonus by all means, but it's really only necessary for one person to use it on 90% of the encounters thus far in the game, IF that. Don't be afraid to break your sets.
Lastly, some comparisons.
Halo of Transcendence
Binds when picked up
Head Cloth
94 Armor
+17 Stamina
+27 Intellect
+22 Spirit
+10 Fire Resistance
+10 Frost Resistance
Classes: Priest
Durability 60 / 60
Requires Level 60
Equip: Increases healing done by spells and effects by up to 48.
VS
Don Rigoberto's Lost Hat
Binds when picked up
Head Cloth
100 Armor
+18 Stamina
+24 Intellect
Durability 60 / 60
Requires Level 60
Equip: Increases healing done by spells and effects by up to 64.
Equip: Restores 11 mana per 5 sec.
Assuming everyone is using the 30% regen in combat modifier, on an emperor type fight you would need a whopping 47.3 spirit, on your HEADPIECE alone, to equal the amount of regen you're receiving from 11/5. That's more than double the spirit on faith, tier 3.
Belt of Transcendence
Binds when picked up
Waist Cloth
65 Armor
+14 Stamina
+26 Intellect
+9 Spirit
+10 Shadow Resistance
Classes: Priest
Durability 35 / 35
Requires Level 60
Equip: Increases healing done by spells and effects by up to 26.
VS
Grasp of the Old God
Binds when picked up
Waist Cloth
75 Armor
+15 Stamina
+19 Intellect
Durability 35 / 35
Requires Level 60
Equip: Increases healing done by spells and effects by up to 59.
Equip: Restores 7 mana per 5 sec.
Again assuming everyone is using the 30% regen in combat modifier, on an emperor type fight you would need 30.1 spirit, on your BELT alone, to equal the amount of regen you're receiving from 11/5. That's a little less than double the spirit on faith, tier 3.
No matter how you spin this, these two items win.
Some fun sources,
http://tinyurl.com/nsjnf
http://www.wowwiki.com/Formulas:Mana_Regen