Check the documentation for
Defines an object that has a user id, roles and can have session data. The interface is implemented both by the global session and the user entity
It's in the
Session namespace for a reason – it represents something that can have session data.
EntityChangedInterface interfaces extended by
UserInterface tell you that it represents a different concept:
Content entities use fields for all their entity properties and are translatable and revisionable, while translations and revisions can be enabled per entity type.
Defines an interface for entity change timestamp tracking.
This data may be useful for more precise cache invalidation (especially on the client side) and concurrent editing locking.
UserInterface represents a full Drupal user account that can have revisions, fields, translations, etc.
I can't tell you which is more fitting for your use case, but I encourage you to think about which of these interfaces is most applicable for the code you are writing.
My instinct tells me that you would use
AccountInterface for something that is minimally "logged in" (e.g. logged in user, a list of users online, someone signed in through SSO) and
UserInterface for anything more than that (e.g. user profiles, associated authors, long-lived data, most things we do in Drupal).