Mastery - more effective active mitigation
Critical Strike - much greater passive mitigation & dps
Versatility/haste - better healing intake, absorb & rage generation
There is a lot to be considered for the value of each stat, and some are better than others depending on the fight, or the entire instance for that matter.
Places where active mitigation is my priority:
Butcher
Imperator
Gruul
Flamebender
Kromog
Places where passive mitigation and dps is my priority:
Operator
Ko'
Tectus
Blackhand
Iron Maidens
Twins
PVP Battlegrounds
Places where healing intake, absorb &/or button mashing forgiveness is my priority:
Oregorger
Blast furnace
Kargath
Brakenspore
Heroic Dungeons & CMs
Shield Barrier, which is one of the most necessary mitigations for a few fights,
is hillariously weak compared to most other tanks ability to handle big magic hits - and that requires some attention. In the fights where it's not too stellar, it's basically worthless and not worth bolstering.
Regemming & chanting for every couple fights doesn't seem viable, so what my raid team tries to do is call in externals for me to deal with Oregorger's Acid Torrent or the cinderwolf's 3rd breath.
I'm currently gemmed & chanted a mix of crit & mastery.
Going through prot warriors of top guilds,
I find 3/5 are gemmed mastery,
1/5 are gemmed crit
& 1/5 are gemmed versa
What's your preference?