Clarifications about Rails and Node.js
There aren't enough differences between Node.js and Rails for it to practically matter.
A lot of what Node.js can do can be pulled off in Rails with things like EventMachine and Pusher. So unless you are really familiar with Rails' limitations, and know you'll be pushing the boundaries, you'd be hard pressed to make something a seasoned Rails developer couldn't do.
Having built apps in Node and Express I can say that they alone aren't enough to make a sexy application. They can seem just as old and stale if you don't have an outstanding frontend UI to facilitate the backend possibilities. Instead of comparing backend servers, I think the real future of doing amazing things is in front-end JavaScript frameworks like Backbone.js that use Express/Rails/Node.js on the backend.
I have chosen to go in the direction of Backbone.js with Rails as my backend API server. Because it's so easy to rapidly create a very nice RESTful backend server in Rails. Rails also makes working with CoffeeScript and precompiling/organizing Backbone code a breeze. There are already decent Backbone.js gems out there for Rails.
The Rails core is also able to acknowledge and embrace the fact that frontend JS MVCs are logically a good next step, and they have been working to strengthen the bond between the two. For those same reasons they have also worked to make Rails an even better API server so that it can work with frontend JS easier. Node.js and Express aren't putting as much effort to coordinate with frontend JavaScript MVCs as the Rails community is.
Being good with a JavaScript frontend MVC and Rails as a backend makes you also great for both worlds in terms of getting a job. You will easily be able to hop onto a Node.js project and add value to that team with your superior frontend experience, and you can also roll with the punches on a Ruby on Rails team and add value to them as well.
As official Node.js website explains it:
Node.js is a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices.
On the other hand Ruby on Rails official website says:
Ruby on Rails is an open-source web framework that's optimized for programmer happiness and sustainable productivity. It lets you write beautiful code by favoring convention over configuration.
Given this I guess that it is more appropriate to compare Ruby and Node.js, but even this is not quite right given that Ruby is programming language and Node.js is NOT. You could probably compare JavaScript with Ruby but I guess that is not what you meant to ask with this question :)
So, for me, key point in understanding what Node.js truly tries to accomplish is well described on Node.js about page. Key Node.js idea (for me) is described in this sentences:
Node is similar in design to and influenced by systems like Ruby's Event Machine or Python's Twisted. Node takes the event model a bit further—it presents the event loop as a language construct instead of as a library. In other systems there is always a blocking call to start the event-loop. Typically one defines behavior through callbacks at the beginning of a script and at the end starts a server through a blocking call like EventMachine::run(). In Node there is no such start-the-event-loop call. Node simply enters the event loop after executing the input script. Node exits the event loop when there are no more callbacks to perform. This behavior is like browser javascript—the event loop is hidden from the user.
What this should enable you, is that you should be able to easily write highly concurrent programs without even thinking about concurrency using JavaScript syntax and callback functions as basic concurrent runnable units.
Your fear that either Rails or Node.js will be gone in a week is unfounded. Rails has a large community and will be around for a very long time even though currently (early 2012) it's getting a bit of hate thrown its way. Node.js is just getting started and has so much attention I don't think it will have any problems getting to the Rails level some day.
That said I've been evaluating Node.js and Rails as options for a project and the reasons I choose Node.js over Rails are:
"The Rails Way" - In my (admittedly limited) experience with Rails it really seems like you either do it the Rails way or you are going to be in for a world of pain. A big part of the Rails way is to use the ActiveRecord model. The advantage of this is that there are a lot of gems that work with your code happily because they know you'll be using ActiveRecord. The disadvantage is you are mixing your data access & model. I am not a fan of this idea so the Rails way for me still seems a bit.. off.
JavaScript is a key part of client side web development and the idea of using it on the client and server is interesting. I'm not super strong at JavaScript and I can't imagine a better way to get better then to have to use it everywhere.
My project has real time communication needs which while I'm sure can be done in Rails there seems to be quite a bit of positive mention on Nodes ability to handle this with socket.io being the front runner option.
At the end of the day no matter which you choose you will have a great time & learn a ton of new stuff that will change how you write code. If you're not on a big time crunch I'd recommend building a small project management tool in both and see which you prefer.
Either way.. Good Luck!