JMeter is great for simple sites or plain HTTP load. But if you have a complex
web site probably including authentication, flow and AJAX, I recommend Selenium.

In fact Selenium is not a load  test tool in nature and is mostly used for
functional/unit testing web  sites. But with Selenium RC you can easily detect
hot spots and the use JMeter (or other tools such as curl-loader) to focus and
resolve these spots.

If you need to perform further tuning on tomcat  itself (usually after you finish
up with application  tuning) I may recommend using tomcat native connector
(APR/TC), optimizing connector threading, and a front-end reverse proxy such as
mod_proxy or Perlbal over AJP/HTTP1.1.