From my experience in running a small PaaS for ruby apps: if you want to have something that 'just works', go for phusion passenger. If you want to take advanta

From my experience in running a small PaaS for ruby apps: if you want to have something that 'just works', go for phusion passenger. If you want to take advantage of threads, go with passenger enterprise. If you want a more complicated setup, for example with varnish and haproxy between nginx and your application servers, thin is rock solid. It runs Rails 2.x-4.x apps without any issues. It's also well suited for handling websockets traffic. If the workload of your apps can take advantage of threads (e.g. lots of external API calls, or lots of time spent in the DB), a server like Puma will show an advantage. Also, if memory is limited, you can handle more concurrent requests.