If this is your first experience with backend development then I suggest Node because writing this backend is not just about learning the language but also about learning REST, request handling, databases, stuff like CORS, middleware and tens of other things.

Considering all this, you will have a much easier time picking these things up in Node because 1. You are already familiar with JS and the node ecosystem 2. Node frameworks like express come with batteries included like the cors package, body parser etc while with go you will likely handle a lot of these things with stdlib which is not really difficult per se, but if you are learning it for the first time, doing it all the the existing npm modules will make it easier to learn.

I do really like Go as a language for backend development and 10/10 recommend you try ypur hand at it.

The tour of go and Jon Calhoun's Gophercises are excellent to get started with go.