TL;DR - Flask for education and simple projects. Django for creating CRUD apps where you just need it to work.
Shameless plug -- I wrote this comparison[0] which shows how "Hello, World" is written in Flask and Django, and goes over some pros and cons of each in more detail. The editors added the "Why Flask might be better" part to the title, which caused most of the discussion about the post to turn into a flame war, but I tried to be as objective as possible (although I do prefer Flask -- another shameless plug for my book[1] Flask by Example).
[0] https://www.codementor.io/garethdwyer/flask-vs-django-why-fl...