Holy Shock criticals give quick casts and mana return through Glyph of Illumination which many pvp players use allowing less spirit stacking, I think the point of stacking crit is getting larger amounts of healing out between CCs, but I still prefer mastery because those shields help a lot as protecting people when I am CCd or just for spellsteal fodder.
Since theres only one set of PvP gear and you will be gemming Power/Resil it just comes down to reforging, which is such a small difference it probably doesnt matter which way you go a whole lot.