The answer to your question is hidden in this answer to a seemingly unrelated question: https://ux.stackexchange.com/a/4350/127933
MS Windows User Experience Interaction Guidelines suggests the following:
“Use the second person (you, your) to tell users what to do.” So use second person for error messages, help, window or page labels, on-page documentation, and other places where the app is telling the user about the user’s content.
“Use the first person (I, me, my) to let users tell the program what to do.” So use first person for buttons, menu items, and other controls where the user commands the app.
This means you can have first and second person on the same page. You can label a list “Your photos” but have a button labeled “Publish my photos.”
This is consistent with what a user learns in the physical world, where the reader of text is “listening” to what the creator or owner of the text has to say, so the reader is “you” (e.g.., “We appreciate your business” on an invoice, “Your speed” on a radar speed sign, and, of course, any instruction, like “RSVP”). However, there is an exception for text artifacts that are handled by the reader as a form of communication back to the creator/owner (e.g., a contract, waiver or oath will begin “I, [PRINT NAME], agree that…”). That is like a command button label.
You can think of your interface as a dialogue between the user and the system. When a user clicks a button, they are telling the system what to do. When a user reads a label, it is the system telling the user what they are shown.
If we transfer this sentiment to your question, the correct answer would be Show more, as it's a command the user gives to the system when clicking the button.