Flarfan79 it's purely cosmetic. There won't be any difference in performance or security as all the information is always available in the REST API anyway.
The only technical reasons I can see for using ID is if you have auto-generated usernames and use display names primarily, or if you use usernames but allow users to change them on a regular basis. By using ID the profile permalink wouldn't change when the user is renamed, at the cost of being a little less pretty.