core-js is the more common of the two in our index: it runs on 19,656 websites against 14 for SimplexNoise.js, and 3 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.
SimplexNoise.js is a library that generates pseudorandom noise functions, designed for use in video games, graphical applications, and demoscene production.
3 sites run both core-js and SimplexNoise.js. That is 21% of the sites using SimplexNoise.js, 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.