For me, it depends on who my second healer is and if they are any good. If I have a good Ana or Moira then I will play Mercy for the mobility. If I don’t have a good player as a second healer, or if I have someone who plays Zenyatta or Brigitte (however talented they are) then I will probably play Moira for lots of healing. It also depends on the team comp. Idk, I guess what I’m saying is that it’s pretty situational and on a case-by-case basis.