Since spellspeed came to the topic again:
http://forum.square-enix.com/ffxiv/t...n-Nexus-weapon
http://forum.square-enix.com/ffxiv/t...28WHM%29/page3

And for some actual numbers:
Piety
A white mage gets 7,9 mp for every piety they have and receive 6% mp regeneration over time out of combat and 2% mp regeneration over time during combat. Piety would provide you more starting MP and a healthy natural mp recovery during combat. The amount you need highly depends on the encounter. For example: BCOB Turn 1 requires far less MP than FCOB Turn 2 (Turn 11). Having more MP to spend allows you to play more aggressively, however.

Keep this in mind, however: Shroud of Saint does not benefit from a larger MP pool

Determination
With previous i110 gear, 25 determination equaled to 10 HP recovery* on a 400 potency spell. This would mean your spells recover more HP for every MP spend and every damage spell you cast will also deal for more. There's no real downside by going for determination.

Critical Hit Rate
Critical Hit Rate has the potential to, much like determination, increase the amount of HP recovered for each MP spend. This situation is quite rare, however. Unless the tank took enough damage that a crit heal won't overheal a significant amount, it won't conserve you any MP at all. On average, having 10% crit rate would increase your healing average by 5%. Stacking crit has some diminishing returns, however. For example: Having a 1% increase on 10% is far more valuable than if you already had 20%. An increase from 10% to 11% would mean 10% more crits over time**. While an increase from 20% to 21% would mean 5% more crits over time.

Spell speed
I'm not gonna bother with this. Read the previously linked topics

* Not exact number, but easier to calculate with
** For easier understanding: 1000 heals, 10% crit would mean 100 crit heals. 11% crit would mean 110 crit heals. 1000 heals with 20% crit would mean 200 crit heals and 21% would equal 210 crit heals. This is respectively a 10% and 5% increase of crit heals.