Turnitin and iThenticate are designed for different types of users. Turnitin is aimed at students and iThenticate is aimed at researchers. Turnitin  is an originality checking and plagiarism prevention service that checks … (the writing of students) … for citation mistakes or inappropriate copying.  It compares the text to its database of student work, websites, books, articles, etc. (https://apus.libanswers.com/writing/faq ).

iThenticate is Crossref’s similarity checker aimed at publishers and researchers.  It identifies unoriginal work and aims to ensure integrity of research. It compares the text to its database of open access articles, books, conference proceedings, pre-prints, encyclopedias, abstracts and webpages