It's roughly 13th Jan 13 being the last part of a discussion on Mastery in t15:
http://elitistjerks.com/f74/t130590-...9/#post2241708 and it's a probable statement, like mine is, I never confirmed it would be Mast>Crit just it probably would be in t15, but PTR bonuses are a little too buggy in tests. This also leads me to believe that Crit>=Mastery currently now, with neither being the out right winner if you stack one or the other, but rather have both at good levels with crit being slightly ahead.
But this is just my take on readings and testings. Someone with better maths than me is needed.