If you've a bit of Python experience, yu know exactly what yu want, & its a fairly standard web application that involves displaying some content, storing some stuff in a database, having users register, & having administrators b able to control what's on the website: use Django.
If you are a Python beginner: use Flask.
If you want to learn more about the inner workings of your application and want the freedom and flexibility to program it in whatever way you see fit: use Flask.
If you want a lightweight application such as a REST API or an Internet of Things application: use Flask.