Pros:

* Flutter is faster. You really notice that for example the animations in flutter are much smoother than in RN

* Flutter has really fast development cycle with hot reload (< 1s)

* Writing UI only once for both iOS and Android

* Better documentation

* Better tooling support in editors (VSCode and Intellij)

Cons:

* The Flutter ecosystem is quite small, there are many lacking plugins

* The Dart ecosystem is way smaller than JS

* RN uses the native widgets, in Flutter they are mimicked and sometimes wrongly, for example the iOS datepicker