Honestly they both suck. But kings suck more. Id much rather have crit or haste or ap buff.
kings is a pretty silly buff now, but still a buff is a buff. Im guessing you were thinking of this from a prot paladin's perspective, and I beleive mastery increases the effectiveness of bastion of light which will make your wogs bigger, more self healing more mitigation, versus slightly more dps. I will have to go with might.