EventMachine vs Node.js

EventMachine has nothing to do with Rails apart from them both being written in the same language. You can get EventMachine as bare as Node.js; all you have to do is not add libraries to your project. In my experience the EventMachine libraries (like em-http) are much nicer than anything for Node. And you can use fibers instead of callbacks to avoid callback hell. Complete exception handling is pretty much impossible in Node because of all the callbacks. Plus Ruby is a nicer, more complete language than Javascript.

I tend towards the "use what you know" (even if it's a heavier architecture). Because of that, I don't see it being quite as simple as "EventMachine vs NodeJS." Mainly, the difference can be summarized as this:

• NodeJS is a framework/language that was written to handle event based programming in JavaScript. That is its driving force. It's not an after thought, or a third party mechanism. It's baked right in to the language. You create callbacks/events because that's how the language is built. It's not a third party plug in, and doesn't alter your workflow.
• EventMachine is a gem in Ruby that gives developers access to some of the goodness of the event based programming model. It's heavily used and well tested, but not baked directly in to the language. Both are locked to one CPU, but with event programming at Nodes core, it still has a leg up. Ruby wasn't written with concurrency in mind.

That said, technical problems can be overcome. The more important questions (from my view) that should guide your decision are these:

• What will your production environment look like? Do you have complete control over the server? Can you host it however you want? Or will it be on a shared system to start with, and then you have to expand on that?
• Do all the developers on your team have the ability to learn a new language very fast? How fast will they be able to understand an event-based language like JavaScript for the middle tier?
• Do you need all of the architecture that Rails gives you (full Testing framework, scaffolding, models, controllers, etc)? Or is that overkill?

There are quite a few technical differences between the two. One is a language, one is a framework. Really, how heavy of a stack you want to run? How much learning will your developers have to do? Do you want a full stack the gives you a lot of niceties, that you may not use, or do you want a bare bones set up that runs extremely fast and concurrent, even though you may have to write extra boiler plate code and learn a new lanugage?

While Rails is not as heavy as some web application architectures, you're still going to need more processor power than you would to handle a similar amount of throughput in NodeJS. Assuming quality code for both systems. Bad code written on either stack is going to prevent the stack from shining. It really comes down to- Do you really want to learn a whole new way of doing things, or utilize your current understanding of Ruby to get things off the ground fast?

I know it's not really a definitive answer, but I hope this helps guide you to a decision!

One thing worth mentioning is the production story. EM, like most Rack stuff, has plenty of testing and monitoring tools available that are well tested, whereas Node.js falls well short in this respect.

At the time of writing, it seems almost impossible to get clear metrics from Node to answer questions like 'Do I need to scale'. There are options starting to form out there from the likes of Joyent, and always the roll-your-own argument, but nothing anywhere near tools such as NewRelic.

Node.js is very good from a performance / configurability point of view, but personally I wouldn't host it in production just yet.