So, for a resource/time-strapped startup, does it make sense to migrate away from Phusion Passenger to Unicorn? The stated advantage of Passenger, multi-tenant deployment, is not applicable to use. Will moving to Unicorn result in increased performance? Or are we better off solving other problems right now?
Your use case is exactly what we built Phusion Passenger for. It could be that applying some configuration could improve startup performance for you, but general runtime performance of Passenger should be on par with a Unicorn solution.
Note that to use Unicorn you would have to configure an nginx instance that proxies for Unicorn. This is where Passenger shines, we already integrate with nginx so there's no extra configuration or management overhead. Passenger is resilient so it will survive any crashes of your application and automatically respawn any failed processes.
edit: The configuration to make your Passenger perfect for dedicated single app servers is:
You can measure your performance and tweak the number, as long as they are equal passenger won't be spawning and killing anymore, making your performance more reliable.
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.
I tend to watch what Heroku is currently recommending for people deploying monolithic Ruby apps on them, since it gives you a good sense of what makes best use of a set memory slice (512MB.) Right now, they suggest Puma.
Moving to Unicorn will probably not increase performance. The performance characteristics of Unicorn are similar to Passenger's. Plus, Passenger has far better administration tools and documentation, so that in the event of problems you can quickly figure out what's wrong.
Passenger works just fine with single-tenant dedicated instances. Just configure passenger_min_instances to the same number of max_instances or max_pool_size, and it'll behave exactly like Unicorn's default settings.
