React Native vs Flutter: what the best option for app mobile development

Do you know React or Flutter?
Do you need to develop something fast? Then use the one you know.
You want to learn something new? Learn the other.

It's all about learning and openness. If you come from Web then ReactNative is closer to you. But also, have in mind thay Flutter's learning curve is quite narrow. You can start learning on real app.

I used to be an Android dev and moved to Flutter 2y ago. Never looked back.

At the end, the users don't care.

"what is the best" is a subjective question. For me, it would depend on what skills you have access to. If you don't know either - both ecosystems are very good.
I have mostly C# / .NET experience, so my apps are built using Xamarin, which compiles down to fully native apps with access to all the native APIs.
I think React Native is pretty much the same.
Flutter is a bit different, in that it's drawing it's UI on a canvas - which if you're not worried about platform-specific UI controls, then that's a great way to go.

I like having access to native APIs, because when I want to know how to do something, I'm not limited to C# xamarin posts on stack overflow - I can find a swift / java solution, and that will work for me :)