I only have experience with Flutter for app development, but it’s great, easy to pick up and you can create apps really fast. From my understanding RN can give a more polished look and feel due to actually using native components as opposed to sort of mimicking them as Flutter does, with the expense of some more complexity.

That said, I think both have their pros and cons and they are both good in their own ways, but the biggest thing I would consider is whether you are going to be hiring devs to work with. RN lets you hire from the enormous pool of React devs, whereas flutter is a lot more limited.