Crit is better than mastery of you are healing very aggressively and limited by mana. Or if people's health doesn't get very low.
Mastery is better if people's health is getting low and you don't have to worry about mana.
Our gearing strategy is based on a simulation of a raid encounter that includes other healers as well. The other healers don't let people's health get really low. You could try making a custom strategy with the new mythic+ script set at a high level and then you would probably see mastery favored more.
You could also try adjusting your play style for the crit build: don't wait for people to get low health: aggressively heal them and make use of that extra mana return.