If we are talking attribute points, MIND will always triumph PIETY due to MIND severely increasing both your healing and attack magic potencies. If you are having MP issues, PIETY is never the answer, especially when you consider that the natural in-combat MP regen is around 1.99% (basically 2%) of your MAX MP, which translates into an extra cast here and there.

To give perspective, at level 60 and with iLvL 240 (919 PIETY) gear, my max MP Pool is 15,142. Moving my 35 MIND Attributes to PIETY (now at 954), I reach to 15,368 max MP pool which is a small 226 max MP increase. If we look at natural in-combat regen, the difference becomes even more apparent.

919 PIETY -- 15,142 max MP -- 302 natural in-combat MP Regen tick.

954 PIETY -- 15368 max MP -- 307 natural in-combat MP Regen tick.

Note: PIETY only effects percentage based MP restoration spells such as Aetherflow (20%) and Assize (10%). It has no influence whatsoever with Shroud of Saints and Luminferous Aether. Also, as we gain more PIETY in future patches and expansions, the 1.99% natural in-combat MP regen gets closer to 2%.

Disclaimer: This post is not anti-piety on gear, so please do not interpret my post as "avoid PIETY as if it were the plague."