This ultimately depends on your situation. I would have said Flutter since it is 1.0 and RN is still in beta. But my main problem here is that you will quickly run into difficulty in attempting to use C/C++ libraries in Dart.

Dart has beta support for native FFI but it is not stable unlike RN. Right now, they instead have 'Platform Channels' which has a significant overhead when passing messages back and forth compared to a native FFI. The issue is still open on Github: https://github.com/flutter/flutter/issues/7053

I can reuse and port my JavaScript/TypeScript projects into mobile very quickly thanks to React Native. Flutter on the other hand, requires learning Dart which depending on your use-case makes sense if it is a new project, not so if you have invested in JavaScript.

Perhaps once Dart gains a production-level C/C++ FFI, then I can seriously look at both Dart and Flutter again.