sweetheart vs sweetie pie

sweetheart

noun
  • A person very much liked or loved by someone, especially when both partners are young. 

  • A person who is always very kind. 

  • A female member of a college or university fraternity. 

sweetie pie

noun
  • A sweetheart; a beloved person. 

How often have the words sweetheart and sweetie pie occurred in a corpus of books? (source: Google Ngram Viewer )