I would setup a virtualbox to choose the OS I might like the most.
As a rule of thumb when presenting linux to others , I would start out with a fedora installation.
Ubuntu isn't really one of the best choises in my opinion. There's too much hype behind it and exploit are common nowadays. But,as always,it is up to you.
Also,since Ruby on Rails is a cross-platform framework , you won't find many problems as it available virtually on anything , thus, you shouldn't find to many hassles if the goal is to deploy your app to the public ;)
Hope this helps. I'll leave the rest to the experts out there 8)