There is an great article for unicorn vs puma
http://ylan.segal-family.com/blog/2013/05/20/unicorn-vs-puma-redux/
Unicorn is a Rack HTTP server that uses forked processes to handle multiple incoming requests concurrently.
- Process management: Unicorn will reap and restart workers that die
from broken apps. There is no need to manage multiple processes or
ports yourself. Unicorn can spawn and manage any number of worker
processes you choose to scale to your backend.
- Load balancing is done entirely by the operating system kernel.
Requests never pile up behind a busy worker process.
- Does not care if your application is thread-safe or not, workers all
run within their own isolated address space and only serve one
client at a time for maximum robustness.
- Supports all Rack applications, along with pre-Rack versions of Ruby
on Rails via a Rack wrapper.
- Builtin reopening of all log files in your application via USR1
signal. This allows logrotate to rotate files atomically and quickly
via rename instead of the racy and slow copytruncate method. Unicorn
also takes steps to ensure multi-line log entries from one request
all stay within the same file.
- nginx-style binary upgrades without losing connections. You can
upgrade Unicorn, your entire application, libraries and even your
Ruby interpreter without dropping clients.
- before_fork and after_fork hooks in case your application has
special needs when dealing with forked processes. These should not
be needed when the “preload_app” directive is false (the default).
- Can be used with copy-on-write-friendly memory management to save
memory (by setting “preload_app” to true).
- Able to listen on multiple interfaces including UNIX sockets, each
worker process can also bind to a private port via the after_fork
hook for easy debugging.
Simple and easy Ruby DSL for configuration
- Decodes chunked transfers on-the-fly, thus allowing upload progress
notification to be implemented as well as being able to tunnel
arbitrary stream-based protocols over HTTP.
For Puma(Newer version of so called server mongrel)
Puma is a simple, fast, and highly concurrent HTTP 1.1 server for Ruby web applications. It can be used with any application that supports Rack, and is considered the replacement for Webrick and Mongrel. It was designed to be the go-to server for Rubinius, but also works well with JRuby and MRI. Puma is intended for use in both development and production environments.
Puma is favourite for speed and parallelism
If you want to see speed comparison then puma is best.
for more you can see http://puma.io/
Thanks