Honestly lol that is a loaded question and i'm going to break down a few things i learned via playing clerics in the betas.
Purifier great single target healer primarily for the hard fights in dungeons when your main purpose would be to keep the tank alive. The buffs they give to allies to mitigate dmg and heal them, and to increase your single target heals are amazing. I wouldn't stay purifier as a main heal spec in group situations because you don't have the capability of keeping as many people up as the other specs.
Sentinel - great for group oriented heals, but their group based spells take A LOT of mana, but the heal effectiveness, and reduction of mana for casting heals are nice and i put the full 5 points into that.
Warden - Great healer for pvp, they have a hot that stacks 4 times, plus a bubble that reduces physical damage and heals your target. I used this in pve as well and had no issues main healing, except for the harder boss fights when single target heals were better for the tank.
What i have also found is putting full points straight up a tree is better than spreading yourself too thin between those two souls. I would even go full 51 points into one healing soul.
I hope those answers your questions...