Above all else if you don't know, your weapon's damage rating is a huge factor in your healing efficacy, and in most cases you should pick a weapon with a better Magic Damage rating over any other factors.

Aside from that for SCH specifically:

MND is the most important stat, followed by Crit (Adloquium is insane with very high crit). Spell Speed and Determination both have their benefits and are relatively small performance changes and both have people that argue over which is better. Piety shouldn't concern SCH too much as Aetherflow should be able to sustain your MP in nearly any fight.

For WHM, Weapon damage and MND are the same priority as SCH. For the rest, it's pretty much a wash and down to opinion or playstyle, there are benefits to all the secondary stats.