Docebo or CornerStone?

In my 12+ years of experience, in which I have worked directly with Cornerstone and Docebo, Cornerstone is the clear winner.  Both offer a sandbox/pilot portals.  Cornerstone also offers a stage portal.  Cornerstone "pilot" portal is your playground to test on what you have licensed and already have configured.  Cornerstone "stage" portal is your playground to test new and upcoming updates, changes, tools, and functionalities for all available Cornerstone modules.

Cornerstone is also much more advanced in training assignments, group assignments, reporting, etc.

You also have an option to license open API access to Cornerstone's APIs which are also more mature in their development than Docebo's.

Both have extended enterprise solutions. Both have custom pages and menus options; however, Cornerstone's solutions are again more mature and advanced in what you can do with them.