What's the difference between Firebase JavaScript SDK and react-native-firebase

According to project description on GitHub:

React Native Firebase is a light-weight javascript layer connecting you to the native Firebase SDKs for both iOS and Android which aimes to mirror the offical Firebase Web SDK as closely as possible.

Although the official Firebase JS SDK will work with React Native; it is mainly built for the web and has a limited feature-set compared to native.

So, you should choose it, for react native platform. This will allow you to use more features and also benefit from using native sdk's (speed, memory consumption, etc.)