Tomcat is the servlet engine.
If you are not using tomcat you are using jetty the build in servlet engine
presumably you downloaded jboss which is combined with jetty as jboss-web. (it's just better hidden).
The tomcat version will do the same thing, but you won't be able to get at the tomcat directory, jboss just uses it.
So if you like tomcat go tomcat otherwise use the default (jetty).