Django is great if you're making a standard cms type website. News feed, blog, etc. There's a bit of a learning curve but it gives you a lot of the scaffolding out of the box (and there are loads of examples to learn from). It has the biggest community (I guess?) so you'll always be able to find answers / libraries / sample code.

Use Flask if you really want to get a website up and running with half a dozen lines of code. You'll have to build everything else yourself; admin, authentication, etc etc - though there are good extensions for a lot of things too.

Thinking about it - if you don't know either of them, spend a day with Flask, by the end of it you should have a fairly good idea of what you get with it. Then run through the Django tutorials. It'll take longer but you'll be impressed with how quickly you can get something fairly complete up and running.

For me, I lean towards Flask (heavily). When you think about what Django does - it's basically a web container with a db/model layer and sensible scaffolding for the views (front and admin). I prefer to use SQLAlchemy (if I'm working with a relational db). In terms of the admin I think that the days of writing admin grid and edit screens that post back and forth to the server are numbered (I use REST and Angular now) - so that's not a bit of Django I use either. By the time you've ripped all that out you may as well save yourself all the config and just use Flask. Having said that - LEARN THEM BOTH. At least just a little bit so you can see for yourself where each one shines.

These days I approach it completely differently. My last two projects have involved a fair amount of complex business logic (and processing work). I've written the libraries I needed first as standalone packages, only introducing Flask as a final step. The web wrapper is just there to provide a web interface (mostly an api) access the db layer and glue it all to my libraries. I'd recommend this approach as it stops you from thinking along the lines of "ok, I'll need to create 3 Django apps, here are my models for each, should I put this code in the models.py or the views.py?" Instead you concentrate on making the real python code to do the heavy lifting, and you architect that sensibly without having to worry about how the web framework would normally prescribe it to be done. You'll end up with far more portable code in the long run.

However, for large web projects, Django beats it hands down. Doing similar work with Flask would require so much wheel-reinvention that it'd be hard to justify your time or your client's money when it's all built quite well already.

Of course it's not perfect, and it's not for everything. But it's a great general-purpose, medium-performance web framework. It becomes a pain on very high-traffic sites (perhaps more so than JVM-based solutions and on par with scaling Ruby frameworks), but if you use it in the right place and/or team up with systems people that know their stuff, it's a powerful tool for quickly building complex websites in a maintainable and intuitive way.

Also Heroku has been mostly a godsend, as I haven't had to spend nearly any time on sysadmin stuff. One thing that caught me early on, not sure if it's a bug or what, but in Heroku, if you use git URLs for some of your packages in pip requirements (which is useful if you need to customize a package or more frequently fix a bug and can't wait for it to be pushed to pypi), Heroku's package cache has trouble knowing when to update unless you a) change the package version in its setup.py (may or may not be a good idea) or b) change the Python version in runtime.txt (kind of a pain as generally that means two deploys for an update, bump down one minor, then back up).

If it's relatively simple and you don't really want to deal with user registration/databases I'd say definitely go with Flask. Django does a pretty good job of those two but until it "clicks" you end up being confused.