If you have little experience with backend development, Django may help you by forcing you to code into a predictable and sensible structure; Flask won't limit you in any way, which is great but It's easier to shoot yourself in the foot and its harder to keep a consistent codebase.

If you're familiar with SQL, it may take some time to adapt to the Django ORM, which is really great but favors ease of use over power.

All in all, with Django you'll fight to override defaults and with Flask to enforce them. I loved both enemies