Versatility is a much better stat in Legion than it was in WoD.
If you aren't healing low health targets, versatility will give you more healing than mastery. What I want to do is calculate some cutoffs where Mastery becomes better than Versatility for different spells to help people decide what stats will be best for them. I know for sure that targets need to be below 75% health. From what I'm seeing, it looks like targets will need to be below about 60-65% health for Mastery to be better than Versatility.