Comparing languages is very different to compare these two SDKs. I've developed apps on both, and personally I've found Flutter significantly more solid and overall just better.
Everything is faster and just works (builds, hot reload etc.) on Flutter whereas on RN a lot of the things break constantly for no apparent reason. And all that weirdness like it might work if you rebuild a few more times in a row. Moreover, a lot of the debugging tools work exceedingly well on Flutter while trying to use even just the memory profiler crashes RN.
Everything is faster and just works (builds, hot reload etc.) on Flutter whereas on RN a lot of the things break constantly for no apparent reason. And all that weirdness like it might work if you rebuild a few more times in a row. Moreover, a lot of the debugging tools work exceedingly well on Flutter while trying to use even just the memory profiler crashes RN.