Hello Simon,
Your question, made me think about this. On referring the api, the following textual explanation for the method was retrieved.
1. getLoggedInClientUser:
Returns the currently logged in client user. This is either an authenticated user or an anonymous user. The anonymous user can be a named one or an unnamed one, depending on the configuration of the UME.
2. getCurrentUser:
Gets the IWDClientUser object of the currently running session. This can be either an authenticated or anonymous client user.
Technically speaking, I always use getLoggedInUser as it gives the user who has logged into the portal. getCurrentUser seems more focused on the session of the application/portal.
Regards,
Sharath