They're both great, but I recommend Flutter. React Native is a bit more mature and developed, but from my experience Flutter was much nicer to work with.
I used Expo when working with React Native, and I hated it.
Flutter I was able to just set up in VSCode, and it had the hot reload feature which made it much faster to prototype UI.
I personally also enjoyed the declarative style of UI design.
I feel like React Native gives much more flexibility and customization, whereas Flutter has premade widgets specifically for Material Design, which look good but offer less customization.