Heroku and Passenger aren't mutually exclusive.[1] Heroku users must choose an app server. Heroku currently recommends Puma,[2] even for apps that aren't thread safe.[3] I don't know why they don't recommended Passenger.

[1] https://github.com/phusion/passenger-ruby-heroku-demo

[2] https://devcenter.heroku.com/changelog-items/594

[3] https://devcenter.heroku.com/articles/deploying-rails-applic...