It really comes down to need. I sync both my calendar and reminders with MS Outlook at work and Google for my personal and both appear on my iPhone. I need my calendar to show where I'm suppose to be and my reminders what I'm suppose to be doing. For example my calendar lists my meetings and I use reminders for my tasks. Some of my tasks are detailed ... Like annual reports and so in the body of my reminder I list how to compile the report so I don't forget next time it is due.
Basically.... Both these applications keep me sane.