Android has a large user base across the world while iOS has a significant user base in North America and Europe. You should first assess where your users are concentrated, and then take the call of whether to go for android or iOS. Another good option is to go for cross-platform app development.
The healthcare app development cost is halved with a cross-platform app as one app works both on Android as well as iOS devices. Thus, you do not need to build separate iOS and Android native apps.