1. Architecture: Sinatra is a lightweight web application framework written in Ruby, while Tornado is a web framework and asynchronous networking library written in Python. They have different architectural approaches, with Sinatra focusing on simplicity and minimalism, and Tornado designed for handling large numbers of concurrent connections.
2. Event-Driven vs Non-Blocking IO: Tornado utilizes a non-blocking I/O loop that allows it to handle many connections simultaneously without multiple threads, while Sinatra follows a more traditional event-driven model using Ruby's threading mechanism for concurrency.
3. Language: Sinatra is built on Ruby, a dynamic and expressive language, whereas Tornado is developed in Python, known for its readability and versatility. This distinction in languages can influence the coding style and ecosystem of plugins and libraries available for each framework.
4. Scalability: Tornado is adept at handling long-lived connections and massive numbers of concurrent clients, ideal for real-time web applications or APIs with high traffic, whereas Sinatra may struggle with scaling to the same level due to its thread-per-connection model.
5. Web Server: Sinatra typically runs on a standalone server like WEBrick, although it can be deployed on other servers, while Tornado includes its own HTTP server, making it self-contained for running web applications without additional server software. This difference can affect deployment options and ease of use for developers.
6. Community and Documentation: Tornado boasts a robust community and comprehensive documentation due to its popularity and backing by companies like Facebook, enhancing its support and resources, whereas Sinatra, although well-established, may have a smaller community and fewer resources available for developers seeking help or extensions.

In Summary, key differences between Sinatra and Tornado lie in their architecture, I/O handling, programming languages, scalability, web server options, and community support.