Unholy Death Knights!
PvP = hit/expertise>mastery>haste>crit
EDIT: Follow socket bonuses.
Red = str/haste
Yellow = str/haste
Blue = str/hit
Most of the time, you'll get around the same amount of strength by following socket bonuses then if you didn't. Plus the bonus secondary stat gain.
Fact is, your pet scales off haste, crit and attack power. Not mastery.
Oh, pve? I'll have to defer to more knowledgeable folks on that score. I think Haste is the favorite, with Crit and then Mastery as the least desired, though I might like to see for myself sometime.
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