In my honest opinion, I have used both Ubuntu 14.04 and CoreOS for the main node for Docker. I would like to suggest CoreOS as the default solution. CoreOS is bare metal and based on the ChromiumOS and ChromeOS.
The difference between Ubuntu and CoreOS is the security and sandboxing of apps. CoreOS uses a fixed minimal install, so basically no crap is added when CoreOS is built by the CoreOS team.
Discourse compiles a lot quicker on CoreOS on the factory installed Docker than Ubuntu.
I would love to see other people opinion about using CoreOS.