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I personally use reminders differently than I do the calendar. Kindof like google tasks to calendar.
Reminders/Gtasks for me is more of errands I need to do or phone calls. I also use it for certain things I need to pick up or accomplish at some point.
Calendars is more for concrete dates. Like appointments or due dates for bills, dinner plans, etc...
As far as the actual capabilities of each being similar, I don't really have much input on that end.