Ionic and React Native serve different purposes; comparing the two isn't entirely fair. Ionic is a web-based cross-platform framework that builds mobile apps using web views. In contrast, React Native creates genuinely native cross-platform apps.
React is the way to go if you simply want to construct an app that can give a near-native experience for the mobile platform. However, if you're going to create an app that can be used on the web, desktop, and mobile, Ionic is the way to go.