I do use none of them now, though own each of them.
Fantastical 2 is really great in quick input of the events and tasks/reminders. Calendars 5 doesn't have natural language input of tasks, unlike events. Though, Calendars 5 is better at managing tasks both visually (better UI) and technically (ease of reordering, i.e.). Also, Calendars 5 has a great week view.
So, if you don't care about structure of your events/tasks and like quick entry of events/tasks - then I would recommend Fantastical 2.
If you like to have a full control over your tasks and events - go for Calendars 5.