Cant really say is it better and i think simming it might be a good idea. But looking at Retribution paladins from top 20 guilds and 9/10 uses it, so i think it is better.
Our gear level has reached a point where Str and Mastery trades at 2:1, meaning 1 Str equals 2 Mastery.
Heroic tanking belt from Shannox has less Str but the massive amount of Mastery it has more than off-sets these values and come out on top as the BiS belt for Ret pallys.
Yes it's BiS because of what Apoc said, so you can have more mastery then crit. Been killing Shannox since week 1 of firelands and still haven't seen that pos belt drop, and 3 people need it.