Gearing for 10m holy has been confusing, it is only this tier that most priests have the ilvl to reach those high haste renew breakpoints. Most fights you're best off using chakra: serenity, but there are some where sanctuary works fine, or chakra switching.
For Chakra: sanctuary mastery will give the biggest output, but mastery healing is delayed, good for sustained damage (i.e. works for pushing new content, pointless overheal when you outgear it). This will be the gearing strategy for many 25m holy priests, so is what most guides point to.
For Chakra: serenity renew rolling, haste breakpoints are important, haste anywhere in between breakpoints is probably better reforged to crit. Again, renew rolling is good for sustained damage. Mastery is by far the weakest stat because neither renew nor lightwell benefit from mastery. Crit is reasonably good for serenity renew rolling - lots of crits on renew ticks add up.
Crit is not so bad for 10m, it's probably what works best for chakra switching and is efficient in terms of heal per mana. As you gear up and outgear content, overheal increases with all that mastery and renew rolling, so casual players might favor crit more.