Puma vs. Unicorn
TL; DR - Just use Puma.
Rails has changed its default server from Webrick to Puma in Rails 5!
It also supports Action Cable (one of the new features from Rails 5) and Basecamp said they're using Puma in production.
I was wondering if Puma has been in fact better than Unicorn so I've read some blog posts about it:
Basically, the answer is yes - it is a bit better, but don't wait for a huge improvement.
It'd be a good idea if you're either about to start a new project and migrating your current project to Rails 5 or newer.