I have little interest in mobile development, though I can see myself having to delve into it in full force in the future. Thus far, I've been able to steer clear of it. That said, if I had to, I would go with the one where there were the most employment opportunities. Right now, it seems that's React Native. Also, the carry over between React and React Native opens the employment scope up a bit more. But if you're in it for the enjoyment, try both and see what you like.

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.