I look at this way. The Reminders app is sort of a To-Do list for simple tasks. It's like writing yourself a note electronically ... Pick up milk on the way home, let the cat out, don't forget to stop at the post office.., things like that.
The calendar app is a more robust app that allows you to share your meetings and appointments with others - if you need to or want to do so, it has the ability to repeat recurring events ( with some limitations), it has the option for multiple alerts (2) and more customization than the reminders app. You can select from and use a number of different calendars in the calendars app as well.
if it were me, I would use the calendars app for appointments,