I feel it necessary to remind (or educate) people that Regrowth stacks best with spell haste. Lifebloom
will never reach the point with current itemization (Max Heal is 2700 /w sunwell gear unbuffed) where stacking spell haste is better than +Heal for HPS.
Now for my opinion...
Yes, I realize with a certain amount of haste you can put out 5 lifeblooms instead of 4, which spreads out raid healing, but I beg you to consider this: a Tree's role in raids is to heal tanks. Keeping this as your top priority, having spell haste gives little benefit to your true role (Only benefit is opening up the option for Swiftmend/NS more often). Raid healing is just icing on the cake and should be treated as such, don't mess up your role in raids by itemization choices. Stack +Heal, don't worry about spell haste. Raid healing for a druid is like trying to ram a square peg in a round hole: .5s longer quick heal that is horribly inefficient, hots that are overwritten which have a true overheal of roughly 75%. Leave maximizing raid healing to the people with CoH and Brain Heal, spend your itemization on what tree's do best: tank healing.
Back to the facts...
As we are all aware spell haste gains greater effectiveness with the more +Heal you have, but at what point does spell haste increase your HPS more than +Heal? Lets get some givens...
+Heal to Spell Haste iLvl Ratio can be found by comparing gems...
[Teardrop Crimson Spinel] vs
[Quick Lionseye]
2.2Heal = 1Spell Haste
Druids have the following talents...
Imp Rejuv 15% Rejuv
Gift of Nature 10% All Heals
Imp Heal Touch 20% Heal Coefficient on HT
Emp Rejuv 20% Heal Coefficient on HoTs
When you take into account talents the base heal for top rank spells are...
Heal Touch 3,247.2
Rejuvenation 1,325
Regrowth (DH) 1,413.5
Regrowth (HoT) 1,401.4
Lifebloom (HoT) 300.3
Triple Stack (HoT) 900.9
Lifebloom (Bloom) 660
When you take into account talents the heal coefficients for top rank spells are...
Healing Touch 1.30
Rejuvenation: 1.16
Regrowth (DH) 0.3188
Regrowth (HoT) 0.918
Lifebloom (HoT) 0.675
Triple Stack (HoT) 2.025
Lifebloom (Bloom) 0.472
With the above facts we can begin to compare spell haste vs +Heal. To have 1% Spell Haste requires 15.76 rating. The equivalent amount of +Heal is 15.76*2.2 = 34.67. So, what is better for HPS? 15.76 haste or 34.67 +heal? It depends on the spell...
Upon taking into account the HPS formula, +Heal coefficient, and spell haste you get this handy dandy formula, which I will plug in for triple stack lifebloom...
HPS of +Heal = (+Heal * Heal Coefficient) / Cast Time
HPS of +Heal = (34.67 * 2.025) / 1.5
HPS of +Heal = 46.8
+Heal Needed = ( (HPS of +Heal * Cast Time) - (.01 * Base Heal) ) / (.01 * coefficient)
+Heal Needed = ( (46.8 * 1.5) - (.01 * 900.9) ) / (.01 * 2.025)
+Heal Needed = 3,022.11
Amount of +Heal for Spell Haste to be better for all druid spells...
Heal Touch 969.15
Rejuvenation 2,324.75
Regrowth (DH) -966.81
Regrowth (HoT) 1,940.42
Lifebloom (HoT) 3,022.11
Triple Stack (HoT) 3,022.11
Lifebloom (Bloom) 2,068.69
Take note that Regrowth's direct heal is a negative number. That means stacking spell haste is better for Regrowth's direct heal with 0 +Heal!
My sincerest apologies if my opinions have offended anyone, that was not my intent. I felt there was a lack of factual comparison between +Heal and Spell Haste.