Screw them both in situations of threat. There are no more bosses that parry gib anymore so Expertise is not needed for survival and vengeance easily covers any threat issues that pop up.
I do switch my trinkets to cap hit for a couple of specific fights where there are interrupts though. Namely Ascendant Council, Omnitron, Halfus, and Nefarion. I would not bother if we did them in 25 man though, but in 10 man having the extra interrupt is key.
My normal mitigation gear puts me at something like a whopping 1% hit and 6 (3 from human) expertise though. I'd get rid of it all if the game would let me.
I think right now the only tossup is on Gems as there are people still gemming full stam and people gemming full mastery and both are succeeding at heroic raid bosses so both are viable, the question is which is better.