Is SQL Server Express good enough for a developer, or should they get Developer edition?

If it is good enough for production then how can it be insufficient in development. And SQL Express is quite capable of handling fair loads (the kind of loads that would have stressed serious hardware just a few years ago).

SQL Server Express does not require licensing but has a smaller set of features.

Developing against full SQL Server (and Developer Edition matches Enterprise Edition) always leaves the chance that you rely on some feature that is not in the production edition.

At the very least all your testing (including unit testing) should happen against the edition to be used in production.

In this question, since a “full” version is being targeted for production then developer edition should be a good match, just be careful of enterprise features if you will deploy against Standard.