core-js is the more common of the two in our index: it runs on 16,603 websites against 45 for Quicklink, and 19 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
Quicklink is a JS library which aims to be a drop-in solution for sites to prefetch links based on what is in the user's viewport.
19 sites run both core-js and Quicklink. That is 42% of the sites using Quicklink, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.