Bolting React onto an existing codebase is probably best because it is more similar to what you'd see in the real world. Most people don't start a React/Node thing at the same time; they'll start integrating React into their frontend early because they can get little React-compatible widgets plugged in more easily than they can introduce backend changes like finding opportunities for Node.

React and Node are not really related other than they're both JavaScript-based, so the skills aren't really co-dependent. Node is used to execute JavaScript locally (in build tools like Webpack, for example), but beyond a small amount of local scripting for builds, they don't really touch (afaik; I have not yet completed a major project with either of them, just used them here and there).