Servlet 2.4 or Servlet 3.0, which to use?

The “New Vaadin Project” dialog in the Eclipse plugin offers configuration for Servlet 2.4 spec and Servlet 3.0 spec.

Which is best for Vaadin 6.6 and 6.7 (beta)?

The latest is not always the greatest. Is there any reason not to use Servlet 3.0 assuming your web server supports it? (Tomcat 7 in my case)