React Native will get rid of bridge costly serialization/deserialization for crutial parts like animations very soon. See talk https://www.youtube.com/watch?v=UcqRXTriUVI If you had already invested in RN, i wouldn't jump to flutter and rather wait. From what I read here on HN, even Android team is kind of skeptical about flutter (but maybe they are also about RN, who knows).

What I personally don't like about flutter is that I have to learn new language (which I cannot use anywhere else), ui components will never look look same as native (specially visible when you would like to mix native part of application with part written in flutter) and far less components and libraries to choose from than RN.

But of course there are some benefits on flutter's side too. Article covers that pretty much.