Mmmm, Crit and Holy. For a long time these two have been BFFs, and Crit was always a priority for Holy Paladins. So much, in fact, that the proliferation of Crit plate in The Burning Crusade and the mad scramble to stack it resulted in the great Illumination nerf of 2007 (to borrow a phrase from BRK). In my gear guide for Haste, some of you asked why I prioritized Haste over other seemingly important stats like Intellect, MP5, or even Crit. Well, I didn't, really. Most Spell Power plate will already have Intellect, so all you really have to do is avoid the ones that don't.
I've never been a big fan of MP5 on Plate, as it is a poor man's substitute for Crit after Illumination was nerfed in Patch 2.1. It isn't specially important now, either, with the help of Divine Plea. Spell Power values only differ by a small margin between items, so I tend not to prioritize that. Haste and Crit tend to be the values that I look for in items because it's easier to pad Spell Power through enchants than it is to raise Haste and Crit. Both are good stats, although nowadays I tend to pursue Haste because I want my heals to land faster. It's extremely useful when healing Death Knights, for example, who tend to take large spikes of damage.