To me it appeared that the gear that drops from the new heroics is mostly build around avoidance.
I went avoidance because the gut feeling that there was more on the gear.
As I'm slowly gathering the T13 equipment, I see a steady incline in mastery. So, I guess I will be switching back soon.
Maybe somebody can shoot on this assumption with raw numbers.
As for what is better, think the situation will change that every time.
I read that tanking groups (as happens a lot in the heroics) would work better with an avoidance build.
Summary: both will work, see what your gear gives you, decide on that.
Good luck!