Almost every new mobile project starts with the same question. Both frameworks can ship excellent apps, so the better question is which one fits your product and your team.
Start with your team
If your engineers already write React and TypeScript, React Native shortens the learning curve and lets you share more with your web product.
Then look at the interface
Flutter draws its own interface, which makes highly custom, brand-led designs consistent across devices. React Native uses native components, which feels at home on each platform.