I had the same question and ended up with Node, just because there are a lot of resources and a big community to help. Also, I can't share code between server and app, so I can't save code by using Dart on the server.
But in the end both can be used and is your choice, none is better.
If you go with Dart, I recommend you to check out https://aqueduct.io/
People tend to ignore it in these conversations (which frankly, is often disheartening), but it’s been used in production, and is nearly done with a large upgrade.
Depends on what you want to do on the server side. For many, it is enough to have a Postgresql database backend, and a very limited amount of other services needed, in such cases Dart is great for server side.
If you rely on many third-party services, chances are that the client API coverage is not that great, and you are better off with node at this point.