Both can support serious products. The better choice depends on your team, interface and integration needs. The useful answer depends on the company’s workflow, current systems, team and cost of staying as it is.
Choose around the product and team
The framework should fit the people maintaining the app, the native capabilities required and how closely mobile shares logic with your web platform.
Where Flutter is strong
Flutter offers a highly consistent interface and a cohesive toolchain. It is often a good fit for operational apps, data capture and products with a distinctive design system.
Where React Native is strong
React Native is attractive when the organization already has React and TypeScript expertise or wants to share product patterns across web and mobile.
Do not ignore release operations
Testing, analytics, store releases, crash monitoring and dependency upgrades have more effect on long-term cost than a small difference in initial build speed.
Make the next step proportionate.
A short discovery, audit or prototype can often answer the highest-risk question before a company commits to a large build. The purpose is to reduce uncertainty, not create a long consulting phase.