Making the same customization to both the unsubscribe_link and unsubscribe_link_and_mail entries seems like the safest approach.
As Lilly noted, which text string gets used is dependent on the unsubscribe_via_email_footer site setting. Testing it on my local installation, the unsubscribe_link text gets added to the email if the unsubscribe_via_email_footer setting is not enabled. The unsubscribe_link_and_mail text gets added to the email if the unsubscribe_via_email_footer setting is enabled.
The default values of each of the text strings seem to create identical links and lead to identical ways of allowing users to manage their email preferences.
The logic seems to be here: discourse/lib/email/message_builder.rb at 2dae20c05434cdb96f59a996167f77a389169c4d · discourse/discourse · GitHub
Based on the description of the unsubscribe via email footer site setting, I’m guessing that it used to cause a mailto link to be inserted into the email:
Attach an unsubscribe via email mailto: link to the footer of sent emails
That doesn’t seem to be the case anymore though.