1. Configuration: EJB3 typically requires a significant amount of configuration in XML deployment descriptors, while REST services are designed to be lightweight and are usually configured through annotations in Java code, making them easier to set up and maintain.

2. State Management: EJB3 provides built-in support for managing session state across multiple method calls, while REST is stateless by nature, requiring clients to maintain their own state between requests, making it ideal for scalable and distributed systems.

3. Protocol: EJB3 relies on the Java RMI protocol for communication between clients and server components, while REST uses standard HTTP methods like GET, POST, PUT, and DELETE to perform CRUD operations on resources, making it more versatile and accessible.

4. Integration: EJB3 is tightly coupled with the Java EE platform, requiring application servers like JBoss or WebLogic to run, while REST services can be integrated into any technology stack and accessed using standard HTTP protocols, providing more flexibility and interoperability.

5. Performance: EJB3 can be heavyweight due to its container-managed architecture, involving additional layers for transactions, security, and resource pooling, while REST services are lightweight and efficient, making them ideal for microservices architectures and high-performance applications.

6. Scalability: EJB3 applications can be scaled vertically by adding more resources to the server, while REST services are inherently scalable by adding more instances of the service to distribute the load, making them more adaptable to changing demands and traffic spikes.

In Summary, EJB3 and REST differ in configuration, state management, protocol, integration, performance, and scalability, impacting their suitability for various applications and system architectures.