Strictly speaking; With no Java EE features your app hardly need an appserver at all ;-)
Like others have pointed out JBoss has a (more or less) full Java EE stack while Tomcat is a webcontainer only. JBoss can be configured to only serve as a webcontainer as well, it'd then just be a thin wrapper around the included tomcat webcontainer. That way you could have an almost as lightweight JBoss, which would actually just be a thin "wrapper" around Tomcat. That would be almost as lightweigth.
If you won't need any of the extras JBoss has to offer, go for the one you're most comfortable with. Which is easiest to configure and maintain for you?
