Both are useful, but they serve different purposes, so a simple rule avoids duplicates.

• Use activities when the action is about a specific record, such as calling the customer on an opportunity; schedule them from the clock icon in the record's chatter.
• Review all activities from the clock icon in the top menu bar, which lists overdue and today's activities across apps.
• Use the To-do app for personal or internal tasks not tied to one record, such as preparing a monthly report.
• If a to-do turns into team work, open it and convert it into a project task so it can be assigned and tracked in Project.
• Managers can share a to-do with colleagues by adding them as assignees.

In CRM-driven teams, the safest habit is: customer follow-ups as activities on the opportunity, everything else in To-do.