No mobile in my main curriculum. A project requiring an app. I tested both seriously.
Flutter
Dart is surprising: strongly typed, expressive. Hot reload is impressive. Widgets are consistent. True cross-platform (iOS, Android, web, desktop).
React Native
JS/TS I already know. React components. Powerful native bridge but sometimes tricky. Expo simplifies a lot.
For a TypeScript web dev
React Native/Expo. Existing knowledge dramatically reduces ramp-up time.
For a new long-term project
Flutter. Rendering consistency and performance justify learning Dart.