Last a note about Ruby, I think the VM is amazing the API is great and programming in ruby you can throw things together blazingly fast and clean, so its also a good choice. Overall though I'd say Go, I think its the best language to come around in a long time and people will adopt it
Node is better for streaming/real-time applications. You can build any type of web app with it, but it's best known for chat/real-time stuff.
That said, I actually love Node and plan to use it as much as possible, over Ruby/Rails/Sinatra.
It's my 10x better test. In general, I think all three are good to know, and each are good at what it does. Ruby is in the 'everything is an object' world, and does it well. It optimizes for programmer speed, rather than execution speed.
Node.js is like being dragged halfway to lisp and functional programming. Learned a lot here, but having callbacks is almost like having to program with continuations all the time.
Go is for systems programming, and it's got its own take at concurrent programming. It's worth looking into.
Either way, just start at one, and eventually do side projects in the others. Can't hurt to know them all.