At this point, the indications are that both Jetty and Tomcat will be supported plug-ins. Tomcat is the Sun reference model for servlet container implementation and will contain most, if not all features of the spec. The cost comes at resource. Jetty is lightweight. In most applications, you may not use many of the Tomcat additionals.
It depends on your needs. At the moment though, I'd stay with Jetty. You face no major web app migration issues to Tomcat so you can do this at your leisure. Unless you have a business need to formalise on Tomcat, of course.