Jetty vs Tomcat vs the alternatives and resource usage of Spring 3 applications?

Have you have had real world experience of Tomcat 6 or 7 and Jetty 6 or 7 with several deployed virtual hosts all running Spring applications? I'd like to hear about your experiences and findings. I want to investigate performance and resource usage under medium load particularly.

How have you or would you test such details in your Spring applications?

What are the actual alternatives to Tomcat and Jetty when running small to medium sized sites? Currently, I'm deploying on Tomcat 6 on a virtual Debian 5 server. RAM is a costly resource for me and I don't have real disk swap, so resource usage is critical. All my apps are Spring 3 based in the same pattern as Spring Roo applications. I also use MySQL 5, but would like to move to PostgreSQL at some point.