TinyMCE is the more common of the two in our index: it runs on 57 websites against 12 for Trix. Counts come from real scans, not vendor claims.
TinyMCE is an online rich-text editor released as open-source software. TinyMCE is designed to integrate with JavaScript libraries, Vue.js, and AngularJS as well as content management systems such as Joomla!, and WordPress.
Trix is an open-source project from Basecamp, the creators of Ruby on Rails.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.