Should users be shown a login page or the same page they were in after their session timer expires?

Currently I am designing a security product for which the session timer is 15 mins after you will be logged out. Should I

  • a) Show the user a small modal which says they have been logged out
  • b) Show them the login page itself

Showing the login page would mean less effort for the users and they can use the login page to get back to where they were.

Atlassian shows a message to the user that they have been logged out.

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How would I go about evaluating this problem? I thought about using KLM Goms but with just an added extra step the results didn't matter at all.