SQLAlchemy gives you a lower level view of your data. That makes a lot of issues with Django ORM just go away (like slow queries for null foreign keys), and makes the ORM much more powerful, making it possible to run queries that would need customizations of the Django ORM. At the price that it is lower level, and your code will be a bit longer.
Both let you escape into bare SQL easily, and Django ORM integrates well with Django, what SQLAlchemy doesn't.
Both let you escape into bare SQL easily, and Django ORM integrates well with Django, what SQLAlchemy doesn't.