Perhaps most significant difference is that with React Native, like the name suggest, is bound to React while with NativeScript you have the option to choose between either TypeScript, Javascript (ES6), Angular or Vue.
Another difference is that React Native is using a CSS-in-JS approach to styling which means you are writing the styling in Javascript objects with props like this: backgroundColor. NativeScript is using CSS which you might be more familiar with.
There are more differences and key factors which is unique to the different libraries that you can explore further. Also, React Native seems more industry standard right now but NativeScript is growing in popularity and if you are working with a hobby-project, it might just come down to is what library you prefer!
I am using React Native at work but have used NativeScript Vue on one of my projects because I prefer Vue. And I don't think your would go wrong with either one of these unless you have some specific requirements from the product owner!