Eighteen months of Django

"However Flask is just not designed for large applications or asynchronous servers. Flask wants to make it quick and easy to write a traditional web application."[1]

Flask is excellent when you just don't need a bunch of the stuff in Django, and don't need the batteries to be included. It's not so much that Flask is suitable to high-complexity apps, but more that it's suited to highly custom web apps. Building a music sharing app based on jQuery mobile and backed by Mongo? You're better off with Flask. But at scale you're going to end up doing something custom.

Django is highly suitable if you need to manage users, have a variety of entities that lend themselves to the admin dashboard (more broadly, that the admin/public app metaphor even makes sense in the first place), and find a plug-and-play package system compelling.

They are both great frameworks, but "complexity" (advanced/simple) is the wrong axis along which to evaluate them.

[1]http://flask.pocoo.org/docs/design/

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.

They just wanted a simple web form, where the sales people could add notes throughout the day and then automatically email those notes to the owner at the end of the day.

I first tried Django and was quickly overwhelmed.. I switched to Flask and with the help of flask-login, flask-wtforms, and flask-mail, it only took me a long weekend to get it up and running.

Just a personal anecdote, but Flask was much easier for me.