if not vs instead

if not

adv
  • Used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the former. 

instead

adv
  • In the place of something (usually mentioned earlier); as a substitute or alternative. 

How often have the words if not and instead occurred in a corpus of books? (source: Google Ngram Viewer )