city manager vs commissioner

city manager

noun
  • A municipal official responsible for the operations of a city, usually appointed by a town or city council rather than elected. 

commissioner

noun
  • An official in charge of a government department, especially a police force. 

  • Someone commissioned to perform certain duties. 

  • A member of a commission. 

  • Someone who commissions something. 

How often have the words city manager and commissioner occurred in a corpus of books? (source: Google Ngram Viewer )