Use Rails if you're working for a client and need some fast cash. If you want to enjoy coding your app and keep maximum flexibility and app speed, use Sinatra. It's easy to build large applications with it, people who say otherwise haven't used it.
Node.js is more like Rack, which Rails and Sinatra is built on, and doesn't really compare. Express.js, which you can use on top of Node.js, is comparable to Sinatra, there might be other frameworks that compare to the bloated Rails stack, google it. If you like coding async Javascript, use Node.js. It's also faster than Sinatra and Rails, but coding in Ruby has it's advantages since you can use lots of really nice ruby gems which makes life easy.
