I don't think Spring even comes into the consideration of which application container to use. Spring will provide a great value to either container. If you have need for the features that jboss provides, then use jboss, otherwise, use tomcat. As you noted, tomcat is a servlet container, and if that fits your needs, then there is no need to use jboss.
JBoss does include a servlet container (tomcat is one of the possibilities) and adds a lot more besides, like the capability to host EJBs, and a whole lot more.
You may also want to use a jboss container if you anticipate you will grow beyond your current needs, and maybe want to add a workflow, ESB, etc.
Since you are already familiar with Tomcat, why not browse through the JBoss site to see what they offer?
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