Let's see what both stats gives and you'll have your answer...
Intellect : More mana, More crit, More regeneration
Spell Power : Boost the spell into being more powerfull.
there you go, you have your answer.
now to gain even more answer as to what you really should be using, for a priest spell power is a lot less potent then it is for the others, which is why a priest is so good at healing. for it needs intellect more then spell power. how do i know that ?
simple, you look at this page...
Spell Power Coeficient
if we take only the healing spells off that list you'll notice that all class except priest benefits a good 100% bonus off of SP when it comes to their healing, while priest on the other hand doesn't gain too much. i was surprised to hear this, but the reality is there.
so you gain your answer there. both are still cool to get, but if you want my opinion, a priest benefits a lot more then the other classes when it comes to intellect. if you'd be doing a shaman or paladins i'd say go with SP, 100% of 100 SP is a great bonus to healing if you ask me !
+22 intellect seems better since it gives you more mana regen thus those who say switch to healing after you lost your mana are wrong in doing so. a staff with lots of spirit on it is good to regen, but having a lot of intellect adds up to the regeneration. thus id say it should be the other way around. use SP gears until you have low mana at which point you switch to intellect to regen what you need while still be able to heal !
but thats just my opinion.
DRUID
Spell Name ------ Spell Power Coefficient
----------------------------------------------------------
Healing Touch --- 161.1% (201.1% with Empowered Touch)
Rejuvenation ---- 37.604% (per tick)(45.125% with Empowered Rejuvenation)
RegrowthDH ----- Approx 53.90% (64.68% with Empowered Rejuvenation)
RegrowthHoT ---- 18.80% (per tick)(22.56% with Empowered Rejuvenation)
PALADIN
Spell Name ------ Spell Power Coefficient
----------------------------------------------------------
Holy Light ------- 166.00% Patch 3.0.9
PRIEST
Spell Name ------ Spell Power Coefficient
----------------------------------------------------------
Power Word: Shield 80.68% Patch 3.0.8
Renew ---------- 188% Patch 3.1.0
*Unfortunetly the lesser healing and the spell Heal are not in this list so i don't know them.*
SHAMAN
Spell Name ------ Spell Power Coefficient
----------------------------------------------------------
Healing Wave --- 161.06% Patch 3.0.3
Seeing how this table of what we have at 19 seems like that i really now wonder about a full renew build.
anyway see what i meant... all other shave an healing ability that makes full use of SP while the priest only spell that makes full use of SP is renew. i'm quite sure the lesser heal and heal aren'T near the 100%.