I think, to achieve a productive day, there has to be a balance between the two. I use Calendar to enforce and keep a habit and To-Do list to do specific tasks. The trick is not to completely fill your calendar and have not more than 3–5 tasks in your To-Do list for a day; becomes overwhelming.
Another interesting process is to have a Weekly Outcome list with Priorities and general tasks to be achieved and completed during the week. It’s implementation required no other planning source to be used, though this process is a little tricky.