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The answer, is yes. I know you did not ask a yes or no question, but allow me to explain.
First of all, you state which should you stack. Stacking (to me) would be asking what should you be gemming, and enchanting. For gems, match what sockets you have. Blue gems = Spirit. Red Gems = Spl Power. Yellow = Int. You can always get the purple or green or whatever and combine, but I just normally keep to it that simple way.
There is no "best 1 stat" for a holy priest, or any class (well, pallies could argue int is best hands down).
Int is my personal favoriate because it gives a bigger mana pool = More heals, it gives higher chance to crit, and a lot of your mana regen abilities will give back more mana if you have a bigger mana pool (hymn of hope for example).
Big mana pool is useless though unless your healing for good numbers. Sp gives your renews a higher tick amount, your Circle of Healing bigger numbers, etc. So rather then casting one heal spell a billion times to heal because you have a huge mana pool, you want to make sure to have spell power so that you don't HAVE to cast that same spell over and over.
Spirit is great because you get mana regen while in combat, and you get spell power out of it via talents, so that kind of combines int to spell power.
So the moral of the story is, every piece is important. It's not wise to stack one stat for a priest, because they all intertwine. Also, the sticky above that is a video (and text if your like me and prefer to read) that discusses all this in much better detail, with better numbers and examples.
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"If the tank dies, it's the healers fault. If the healer dies, it's the tanks fault. If the dps dies, it's their own damn fault!"
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