Next.js is the more common of the two in our index: it runs on 5,127 websites against 629 for Ruby on Rails, and 12 sites use both. Counts come from real scans, not vendor claims.
Next.js is a React framework for developing single page Javascript applications.
Ruby on Rails is a server-side web application framework written in Ruby under the MIT License.
12 sites run both Next.js and Ruby on Rails. That is 2% of the sites using Ruby on Rails, 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.