"should I decide for Dart or node.js" - This question makes no sense. Node just interprets javascript code and runs it but also has something like modules.
Dart ist a language that was developed after JavaScript and brings a lot of new features. If you enjoy coding javascript code, why would you want to learn Dart then? ;)
I personally never liked JavaScript because there was no real OOP. With Dart there is OOP and much more!
The future will show if Dart will be adapted by firefox, IE etc - but as far as I know Microsoft tries to adopt a replacement for javascript as well - the future will tell. Right now Dart looks really strong and makes a lot of sense to me.
Though Dart can also run on server side and has a lot of cool features and there are lots of packages on http://pub.dartlang.org
In the end you still have no disadvantages from coding in Dart because it is much better and sometimes js code compiled from Dart might be even faster then directly coding in javascript. So you have decide yourself what you like more. As a passionate programmer I'd always prefer Dart over javascript - no doubt here!