Tomcat is actually the servlet container used with JBoss so you're using Tomcat even when you use JBoss
It really depends on your usage scenario, if you are just running the web app then Tomcat is more lightweight and faster to startup, so you could argue that is better.
However, if you want to use the portlet version of the app then you have to use JBoss (or any other JSR168 compliant portal server) as Tomcat does not support portals.