Yoast SEO is the more common of the two in our index: it runs on 5,084 websites against 31 for Advanced Custom Fields, and 28 sites use both. Counts come from real scans, not vendor claims.
Advanced Custom Fields is a WordPress plugin which allows you to add extra content fields to your WordPress edit screens.
Yoast SEO is a search engine optimisation plugin for WordPress and other platforms.
28 sites run both Advanced Custom Fields and Yoast SEO. That is 90% of the sites using Advanced Custom Fields, 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.