The best stam enchant for shields I see only adds 18. After talents for a prot warrior or paladin, that's roughly 200 health.
Both of those classes have talents to boost block value by 30%, so the Plating will mitigate 52 damage every block. Seems to me that you get more value out of this than you would with 18 stam. The block value is going to add damage(and threat) to Shield Slam and Shield of the Righteous as well.
This is the only slot where you're choosing between BV and stamina, so it's easy to add stam somewhere else.
As a tanking paladin I block pretty much anything that isn't a parry/dodge/miss, so I'm taking the Plating.