You can just upgrade or replace the JSF version to whatever whenever you want. Mojarra even provides a Glassfish updater tool. Check their homepage:
http://javaserverfaces.dev.java.net About Tomcat v.s. Glassfish: I myself like Tomcat when it comes into development without the need for
EJB's. Glassfish starts/restarts and debugs very slow, incredibly slow. Tomcat is small, lightweight and very quick. It starts and restarts within three seconds. But when EJB comes into picture, you may better consider
JBoss AS, which is based on Tomcat, but has an EJB container on its top. Not as fast as Tomcat due to the EJB thing, but it's still faster than Glassfish.