Have a look at my HealCalc site in my signature (or let me know if it doesn't work for you), you should be able to pull your stats from armory. Then when you hover over a spell, you will see on the right side a stat comparison on how much benefit that spell gets from each stat. You will see then that spirit shell and PW:S scale extremely well with mastery, while almost all spells scale really well with crit (but not quite at the mastery's level). For myself a large part of my healing comes from atonement and cascade, so I opt for a heavier crit build. Crit still benefits spirit shell a lot but not quite at the mastery level.
Basically: On non-shell non-poh heals, crit is worth close to 0.9 of spellpower, while mastery is closer to 0.2. For non-shelled poh, crit is worth 0.45 of spellpower, mastery is about 0.37. For shelled spells mastery is worth about 0.9 of spellpower, crit is closer to 0.5 (0.4 for poh shelled, 0.6 for others shelled).
I think emphasizing either crit or mastery are both viable choices and would be fairly close in the long run. Crit offers more regular shielding in the form of more frequent DA procs, mastery emphasizes "gimme lots of shielding right now!", by making PW:S and shells more powerful. At the end of the day I don't think you can go wrong either way. And it does all depend on your spell distribution at the end of the day.