1. Architecture: Restlet follows a RESTful architecture, using standard HTTP methods like GET, POST, PUT, and DELETE, whereas SoapUI is specifically designed for testing SOAP web services, which follow a protocol-based architecture.
2. Protocol: Restlet uses HTTP protocol for communication, making it more suitable for web-based services, while SoapUI uses SOAP protocol, which is more beneficial for enterprise-level applications requiring formal contracts.
3. Interoperability: Restlet can work with a variety of data formats like XML, JSON, and HTML due to its flexibility, whereas SoapUI is primarily focused on XML data exchange, limiting its interoperability with other formats.
4. Tool Flexibility: Restlet is more lightweight and flexible, suitable for simple service testing and development tasks, while SoapUI offers advanced features like data-driven testing, security testing, and mock services, making it more comprehensive but potentially overwhelming for simpler tasks.
5. Popularity: Restlet is less widely adopted compared to SoapUI, which is a more established tool in the industry with a larger user base and community support. This can affect the availability of resources and assistance when working with either tool.
6. Learning Curve: Restlet may have a lower learning curve for beginners due to its simplicity and focus on RESTful principles, while SoapUI, with its specialized features and protocols, may require more time and effort to master.

In Summary, Restlet and SoapUI differ in their architecture, protocol, interoperability, tool flexibility, popularity, and learning curve.