Just Look at a site like FFLogs which records healing stats. In cast majurity of logs where AST does less total DPS they also out healed the other healer. They had lower DPS because they healed more instead.

Certain content, of course, will favor a specific class. If a group stands very close together and a lot of AOE healing is needed. white mages can do better with Cure 3.

Regarding Medica 2, it is only a tiny bit better after considering the AST stance bonus and is not something that can be spammed. It only needs to be cast every 30 seconds for 100% uptime. That doesn’t make up for Helios, Asp. Benedict, Benedict 1, and Benedict 2 being strongest in their respective category. Those can be spammed every 2.5 seconds so the impact has the potential to be much higher when a lot of healing is required.