Express.js and Nest.js are both popular frameworks for building server-side applications with Node.js. Both frameworks provide a lot of functionality out of the box, making it easy to get started with building your application. However, there are some key differences between the two frameworks that may make one a better choice for your particular use case.
 

1) Architecture

One of the main differences between Express.js and Nest.js is their architecture. Express.js is a minimalistic framework that provides a minimal set of features out of the box. It is designed to be flexible and easy to use, and is often used as a foundation for building custom web applications. Nest.js, on the other hand, is a more opinionated framework that provides a more structured approach to building web applications. Nest.js is heavily influenced by Angular, and is designed to make it easy to build scalable and maintainable applications.

2) TypeScript

Another key difference is the language used. Express.js is written in JavaScript, while Nest.js is written in TypeScript. This means that Nest.js offers stronger type checking and other benefits of using TypeScript.

3) Performance

In terms of performance Express.js is considered faster in terms of initial loading and boot up time compared to Nest.js. But Nest.js is more suited for building larger and more complex applications.

4) Unit Testing

Unit Testing is faster and easy with NestJS as its CLI inculcates a default testing environment configured in Jest. It creates a spec file when a service, interceptor, or controller is created which includes an auto-generated testing bed code. Hence, for unit testing, you do not have to write extra code. 

In summary, Express.js is a minimalistic framework that provides a flexible and easy-to-use foundation for building custom web applications. Nest.js, on the other hand, is a more opinionated framework that provides a more structured approach to building web applications. It's well suited for building scalable and maintainable applications, but it may be more complex and slower than Express.js. The choice between the two frameworks will depend on the specific requirements of your project and your personal preferences as a developer.