1. Request handling: Koa is more low-level and allows for more control over request handling, including the ability to directly manipulate requests and responses. On the other hand, Slim provides a higher level of abstraction for request handling, making it easier to work with HTTP requests using convenient methods and features.
2. Middleware: Koa uses a more modern and streamlined middleware approach, where middleware functions are composable and can be used in a more flexible manner. In contrast, Slim follows a more traditional middleware pattern, with middleware being added in a linear fashion to the application pipeline.
3. Error handling: Koa has a more robust error handling mechanism, allowing for the easy catching and handling of errors within the middleware stack. Slim also supports error handling, but the approach is more simplistic compared to Koa.
4. Flexibility: Koa is known for its flexibility and minimalistic design, allowing developers to build applications according to their preferences and needs. Slim, while still flexible, provides more structure and conventions out of the box, making it easier for developers to get started quickly.
5. Async/Await support: Koa has native support for async/await, making it easier to work with asynchronous code and avoiding callback hell. Slim, on the other hand, requires additional libraries or custom implementations to fully support async/await.
6. Community and Documentation: Koa has a smaller but active community, with comprehensive documentation and a focus on modern web development practices. Slim has a larger community and more extensive documentation, making it easier to find resources and support when working with the framework.

In Summary, Koa and Slim differ in their approach to request handling, middleware, error handling, flexibility, async/await support, and community/documentation.