I mean every healer can use a little haste right? The most explained answer will be rewarded!

"Twin Disciplines and Darkness are direct HPS increases, so if you don't have mana issues you're best off spending your points here. However, if you do have mana issues, you may instead opt for Twin Disciplines and Mental Agility. If you really cannot sustain your mana throughout a fight, Mental Agility and Veiled Shadows might be something to try out."

Straight cut and paste from here: http://www.tankspot.com/showthread.php?56757-Cata-Holy-Priest-Guide-%284.0.3%29

I always look up the builds on the wow forums before deciding on mine, people there spend a lot more time weighing up the pros and cons of each build.

A build from tankspot is a fairly safe bet.