Log in page field variables

For the login page fields (email, password) should I create and assign page variables or data variables?

The recommended way seems to be Page Variables (e.g. credentials), as you only want to use and store them on that login page and not persist the plaintext password throughout the app.

If you need to persist the email address for some reason, you could persist that additionally in the appVars but the assumption is that the success response from logging in would provide you all the details about the user, which you could/should persist at the app level.