head-to-head vs one-on-one

head-to-head

adj

  • Direct, one-to-one. 

adv

  • Directly, one-to-one. 

  • Facing one another. 

one-on-one

adj

  • Involving direct communication or competition between two people. 

  • bijective or injective 

  • Involving one attacker and one defender. 

noun

  • A contest involving only one player on each side, especially of an activity often involving teams. 

How often have the words head-to-head and one-on-one occurred in a corpus of books? (source: Google Ngram Viewer )