As people have said before, it all depends on what you want to do. Django is a fully featured web framework with everything you'd want and a kitchen sink. Flask, on the other hand, is a lot more like legos.
A simple example would be ORMs. Django comes with its own ORM which is its own beast. With Flask, you have the choice to either use an external ORM like SQLAlchemy or just use plain SQL directly from your app.
Now coming to what you should learn. That again depends on you. If building websites and prototyping ideas fast manner is what you need then I'd suggest Django since it provides you with all the tools and parts required to build a web app. On the other hand, if you want to understand the different parts which make up a web app, then Flask would be the way to go.