As both Reminders and Calendar notify you of upcoming events in the same way, it really makes no difference which program you use. I suggest that you classify reminders as reminders, and use Reminders. Classify events as events, and use Calendar.
In other words, "get cheese next time you're at the grocery store" isn't really a good idea to clutter up your calendar with. "Dentist appointment at 9:30 on Tuesday" is a one-time thing, so it's not really very sensible to make that a reminder. It's a Calendar event that needs an alarm.
Perhaps Siri can be used to help you sort this out automatically. Telling Siri to "remind you" of something will automatically put it in reminders, with an alert ahead of time. Telling Siri "Add 'meet Bob for lunch' to my calendar for next Tuesday at 12:30 pm" puts it in the calendar instead.