Nintex Newbie

What's the difference between using an "Assign to-do task" and "Assign Flexi task"?

Should I be using "Assign to-do Task" or "Assign Flexi Task" to achieve my goal?

My goal is to create a task and receive a reminder that this task has been read and accepted. From there on, reminders will be sent every 3 days or so until the due date or until the assignee changes the status to complete. If there are any updates to the status or task itself, an email notification will be sent to the creator of the task. I'm pretty new to Nintex Workflow 2013 so any advise on how to do this with step-by-step directions would be appreciated. Thanks!