Definitely Node.js.
You're going to learn and master
Javascript which permit, nowadays, server-side or client-side developement. You will be able to do great things on both side. Javascript is getting more and more required in web-applications or websites, since Javascript engines are constantly improved.
We all know that Javascript is a must have on client side, intro was just a reminder, but what about server-side ?
First, you'll see that, node.js is mono-threaded.
WTF ? Monothread ?! OMG This thing sucks ! Everything else is multi-threaded so they are more powerful than nodejs !
Well, i have to disagree. The point is we don't care about what techs/libs are used for handling clients/request, but we do care about
how it's managed and
why it's managed this way. Node.js is an evented i/o system. It mean that he will handle requests as events without creating bunch of unuseful threads.
Evented server are
way more powerful with fast executed requests than threaded servers. And with slow requests, it has exactly the same ratio than threaded server. ( Everything explained & tested here :
Threaded vs Evented Servers )
Why ? Overall because of forking/thread creation is a really painful operation. NodeJS do create threads, but less than standard threaded servers (as Apache server for exemple).
The second point is that as a client-side Javascript master (and i hope you are / will be), coding in NodeJS will be extremely easy and painless. That's a point dev companies do love right now, you can develop for client & server both ! You will just have to keep a tab with NodeJS api openened and CTRL+F what you want to do. NodeJS provide a lot of great/fast modules out of the box (only server-side as is part of NodeJS API) for managing file system, forking/managing nodejs instances, streams and more !
Last point, NodeJS is already loved by a big and great community. There is something called
NPM (for Node Package Manager) who permit to install/download external nodejs modules for doing a lot a things. (ExpressJS is one of the must have module when dealing with RESTful application / websites ).
Please take a look at all little things I explained, Google it or anything, it will cost you 10 minutes but you'll learn A LOT (and i'm sorry for my approximative english).
PS: I will be glad to answer to any question. Just send me a MP or add me on Skype : Shynoshy.D.Live - For your information, i've been working on a company doing only Javascript / NodeJS and using a lot of librairies (async / backbone.js / amber.js and a lot more ) for one year. I know what i'm talking about.
Have a nice day !