User_id is a unique identifier that is linked to a user that sign up on your website.
Tracking the user_id is more useful because while other important user attributes such as email address are subject to changes, the user_id will always be the same.
This means the user profile will be linked to the user_id permanently, therefore maintaining a consistent user record in Gist even if the user changes their email address
Another reason to use the user_id is if you are very concerned with user privacy and do not want to send an email address at all. But remember that if you don't send the user’s email to Gist, we won't be able to send email messages for you, whether it is auto messages or drip campaigns.