Both technologies offer a single codebase for iOS and Android; the choice is made according to project conditions.
Where Flutter is preferred: Demanding animation and custom interface requirements, consistent cross-platform appearance in field applications, offline-heavy scenarios.
Where React Native is preferred: Existing React competency within the organisation's team, sharing of components and business logic between web and mobile, and the need to draw on the broad JavaScript ecosystem.
Whichever route is chosen, backend services, notification infrastructure and the store publishing process must be planned with the same rigour. The success of an application is more often determined by the quality of these layers than by the mobile code itself.