167 (Mirror) vs. 171 (Mark) static AP buff. 84 crit rate vs. 124 hit rate. Remember you're most likely just spellhit-capped thus hit above that rating is still no waste (whitehit melee miss chance = 27%). however i would suggest you to take Mark of Supremecy anyway because you can pop it before fireelem (which will snapshot you stats when summoned) and time it well with your other trinkets proc. furthermore it looks better when you try to get into PUG, because it provides you a nice chunk of Gearscore.
EDIT:
another reason to take it over Mirror - it allows you to have more "moving-space" in terms of you spellhitcap. while apparently you would be spellhitcapped with just 14%/13%(in Raids.. / ..Alliance..), its highly suggested to get around 450 - 500 hitcap because it will increase you whitedmg which is between 30-50% of your overall dmg.
5 years later i can tell Mark is better choise. Why ? Cuz Mirror proc once every 15 mins...
This thinker nor work this way. He proc on hit, but for some reason, here proc on crit...
I spend 20 mins on dummy and proc 2 times on 23% crit chance. It's ridiculous.
Otherwise this is best starting trinker from vendors.