Next.js is the more common of the two in our index: it runs on 5,505 websites against 83 for Phusion Passenger, and 2 sites use both. Counts come from real scans, not vendor claims.
Next.js is a React framework for developing single page Javascript applications.
Phusion Passenger is a free web server and application server with support for Ruby, Python and Node.js.
2 sites run both Next.js and Phusion Passenger. That is 2% of the sites using Phusion Passenger, 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.