Flutter is an open source UI development framework developed by Google. It enables developers to build natively compiled applications for multiple platforms (iOS, Android, web, desktop) using a single codebase. Flutter is known for its rich set of customizable widgets, high performance, and fast development speed through features such as Hot Reload.

Kotlin Multiplatform (KMP) is a set of tools that enables developers to write common code in the Kotlin language for multiple platforms (iOS, Android, web, desktop). It facilitates the reuse of business logic and code while allowing native UI implementation and full access to platform-specific APIs.

The choice depends on your project needs. If you need rapid cross-platform development with a consistent UI/UX, choose Flutter. If you need a platform-specific UI and access to the latest native features, or if you’re migrating a native application, KMP is a better choice.

Kotlin Multiplatform generally provides better performance for native functionality because it uses platform-native rendering. However, Flutter offers high performance for most applications, especially those focused on UI/UX, thanks to its own rendering engine.

Kotlin Multiplatform is more suitable for projects that rely heavily on native functionality, as it provides direct access to platform-specific APIs and supports platform-specific UI.

Flutter is better suited for startups with no existing mobile apps. It offers rapid development with a single codebase, making it cost-effective and ideal for getting MVPs or proof-of-concept projects off the ground quickly.

Flutter is generally cheaper because it uses a single codebase across platforms, reducing development time and costs compared to Kotlin Multiplatform, which requires more platform-specific work.

Choose Flutter if your project prioritizes a consistent UI across platforms, rapid development, and a single codebase. Choose Kotlin Multiplatform if your project requires native performance, extensive use of platform-specific features, or if you’re migrating from an existing native application.