1. Purpose: Hystrix is a latency and fault tolerance library designed to isolate points of access to remote systems. In contrast, Rails Spring is an application preloader that significantly improves the startup time of Rails applications by keeping the application in memory between invocations.
2. Language: Hystrix is primarily used in Java applications to handle distributed systems interactions, whereas Rails Spring is specifically tailored for Ruby on Rails applications to optimize performance.
3. Functionality: Hystrix provides circuit breakers, fallbacks, and monitoring for distributed systems, ensuring system stability and reliability. Rails Spring, on the other hand, focuses on speeding up the initialization of a Rails application by preloading it in memory.
4. Community Support: Hystrix is a widely used library in the Java ecosystem, with an active community providing support and updates. Rails Spring, while also popular in the Ruby on Rails community, may not have the same level of widespread adoption as Hystrix.
5. Integration: Hystrix integrates seamlessly with various Java frameworks and libraries, making it easy to incorporate into existing systems. In comparison, Rails Spring is specifically built for Ruby on Rails and may require more effort to integrate with other frameworks or applications.
6. Runtime Environment: Hystrix is primarily used in microservices architecture where resilience and fault tolerance are crucial, while Rails Spring is geared towards optimizing the performance of monolithic Rails applications.

In Summary, Hystrix and Rails Spring have key differences in terms of purpose, language, functionality, community support, integration, and runtime environment.