Which one between Phoenix and Rails should I choose for our next project?

We're starting a new project, version 2 of an application. Number of online users grows upto 8K at night.

The application is a social networking application with things like viewing other users profiles, liking photos, sending messages, live chat, etc. We'll deploy this application to a single dedicated server, or Linode VPS.

We have two options, Rails and Phoenix. Rest of the team (3 back-end guys) is equally familiar with both, only I'm a bit inclined towards Rails.

Which one of the two frameworks do you recommend?