Choose around the demo boundary
The useful question is not which framework is better. Ask what the demo must prove and how much product infrastructure it needs before a user can understand the idea.
A model playground with two endpoints has a different boundary from a multi-user application with permissions, saved projects, and an editorial interface.
When FastAPI keeps the idea visible
FastAPI is a strong default when the backend mainly validates typed input, calls a service or model, and returns a response. Its small surface area makes request flow easy to inspect during an experiment.
- The frontend and backend are deployed independently.
- Most behavior lives in Python services rather than database-backed screens.
- Interactive API documentation is useful during development or a demo.
When Django avoids rebuilding a product shell
Django earns its larger starting footprint when authentication, relational data, forms, permissions, and administration are already part of the demonstration. Those features are expensive to assemble repeatedly around a thin API.
Start with the smallest boundary you can defend. Moving to a larger framework later is often cheaper than hiding the original idea behind infrastructure it did not yet need.