rather vs that is

rather

adv
  • Used to introduce a contradiction; on the contrary. 

  • Somewhat, fairly. 

  • Introducing a qualification or clarification; more precisely. (Now usually preceded by or.) 

  • Used to specify a choice or preference; preferably, in preference to. (Now usually followed by than) 

verb
  • To prefer; to prefer to. 

that is

adv
  • In other words; that is to say. Used to introduce a clarification or explanation 

How often have the words rather and that is occurred in a corpus of books? (source: Google Ngram Viewer )