.NET core, no question. NodeJS ecosystem is brittle. Yes there are a lot of libraries ... mostly unmaintained and low quality. Quantity isn't quality. .NET core is moving extremely fast and is backed by Microsoft. There is no debate here. And frankly , do you really want to use a language that is as weakly typed as Javascript to write mission critical code ? That's madness. The JS ecosystem is nowhere near .NET's. And C# is way more enjoyable as you can benefit from static typing and way less verbose than Java.

EDIT: A great thing is that you can directly interact with .NET core maintainers, which is definitely a plus. They are paid to write the framework so they are doing this fulltime, unlike people who maintain this or that nodejs library. Just look at the state of Express for instance... it's just rotting since TJ abandoned ship. no, NodeJS ecosystem is definitely a mess.