1. Scalability: NGINX Unit is designed for scalable, microservices-based architectures, providing the ability to scale horizontally by adding more instances as needed. It utilizes a decentralized architecture, allowing each instance to run independently, which ensures high performance and efficiency. On the other hand, Passenger is designed for traditional, monolithic applications and offers a single, centralized application server. While it can scale vertically by adding more resources to the server, it may not be as suitable for highly scalable architectures.

2. Supported Languages: NGINX Unit supports a wide range of programming languages, including Python, Go, PHP, Ruby, Perl, JavaScript (Node.js), and more. It offers a flexible and modular approach, allowing developers to use their preferred language and frameworks. In contrast, Passenger primarily focuses on Ruby applications, providing a mature and optimized environment for Ruby on Rails applications. It may not have the same level of flexibility and support for other languages as NGINX Unit.

3. Configuration: NGINX Unit uses a dynamic configuration model, allowing changes to the configuration to take effect immediately without the need for restarting the server. It provides an extensive set of configuration options, enabling fine-grained control over the behavior of applications. Passenger, on the other hand, relies on a static configuration file that needs to be reloaded or the server needs to be restarted whenever changes are made. This may limit the flexibility and agility in managing the application's configuration.

4. Deployment: NGINX Unit offers easy and flexible deployment options, supporting various deployment models such as containerized environments (e.g., Docker), cloud environments (e.g., AWS, Azure), and bare-metal servers. It allows applications to be deployed independently, providing isolation and flexibility. In contrast, Passenger is primarily designed for deployment within a web server (e.g., Apache or NGINX) as a module or integration. It may require additional configuration and setup to integrate and deploy applications.

5. Load Balancing: NGINX Unit includes built-in load balancing capabilities, allowing it to distribute incoming requests across multiple instances of an application for improved performance and availability. It provides various load balancing algorithms and can automatically detect instances that are unavailable or overloaded. In comparison, Passenger does not offer built-in load balancing capabilities and relies on external load balancers or web servers for distributing traffic.

6. Monitoring and Management: NGINX Unit provides a comprehensive set of API endpoints and tools for monitoring and managing applications. It offers real-time metrics, logs, and allows programmatically controlling application lifecycle. Additionally, it integrates well with existing monitoring and management systems. Passenger, on the other hand, may have limited monitoring and management features, depending on the web server or environment it is integrated with.

In Summary, NGINX Unit and Passenger differ in terms of scalability, supported languages, configuration flexibility, deployment options, load balancing capabilities, and monitoring/management features. While NGINX Unit excels in scalability, language support, flexible configuration, and deployment options, Passenger provides optimized performance for Ruby applications and may be easier to integrate with existing web servers.