This page explains the differences between the two extensions and their use cases.
The axe DevTools Extension (formerly known as the axe Extension), incorporates the latest in accessibility trends and technologies. It can be used to quickly and easily scan a whole or part of a page for accessibility defects using the latest version of axe-core. In addition to automated testing, it includes Intelligent Guided Testing which allows users to catch up to 80% of all accessibility defects on a page.
The axe DevTools Extension is available for Chrome, Edge, and Firefox, but functionality such as saving, exporting, and Intelligent Guided Tests are only supported on Chrome and Edge at this time.
Formerly known as the WorldSpace Attest Extension, the axe Expert Extension is often used in conjunction with axe Monitor in order to upload issues found with the extension and for recording and uploading scripts. It is also use by axe DevTools customers to scan their sites for accessibility defects and then learn how to fix them.
The axe Expert Extension is being replaced. Read more about the axe Expert replacement here.