Projects can have ’normal’ tasks as children which can have their individual due dates, due times, repeat patterns etc.

On the other hand, all child tasks of a Checklist share the due date set for the parent task. Child tasks cannot have their own due dates or due times. When synced, child tasks will show the same due date you had set for the parent task. Also, with checklists, child tasks cannot have recurrence / repeat patterns set to them.

So you would normally use checklists when all sub-tasks are really tied to the main parent task in terms of time and date. Similarly you would use Projects when child tasks can be due on different dates/times and can repeat on their own.