They're not exactly doing the same job. Flask is a micro-framework which you can build/prototype things fast but lacks authentication, models, ORM. Django, on the other hand, has everything you need in a web app with batteries included.

This also effects learning curve. You write Python when you build with Flask, but you write Django when you build with Django :)

I use them both for different purposes. If I need to build a project that requires multiple modules and lots of tables/models, I would probably go with Django. On the other hand, if I need to build/prototype stuff fast, I would go with Flask.