Is your app mostly presentation, database (CRUD type maintenance) , or lots of back-end logic?
For mostly presentation I would go Python/Django (or even php).
For simple CRUD RoR has it licked.
For anything with heavy back-end logic, or, which uses an existing legacy database I would go for the java servlets using Spring with Freemarker, Ibatis and Tomcat or Jetty.
P.S. Don't worry about scalability performance -- nothing will scale as well as the Tomcat/java solution.