It depends on what type of project you want to work on.
To put it very briefly, Django has far more features and is set up to do a great deal more out of the box. Flask is more simple, but very extensible if you use the right packages.
Personally I prefer working with Flask because my projects are usually REST API backends. My frontends are usually made in Node. But if you're doing full websites, and want it all in Python, then you'd prefer Django.