I don't recommend any particular one. If you want just JSP/Servlet support, both suffices. If you want more than that (e.g. anything provided by the Java EE API which is much more than alone JSP/Servlet), then Tomcat simply don't suffice without manually adding a bunch of components on top of that to comply the complete Java EE API like JBoss AS is doing.
In (Eclipse) development terms, Tomcat is a quick starter and restarter, takes about 3 seconds. Glassfish is a slow starter, takes 5~10 seconds for a simple webapp, but it is an extremely fast hotdeployer (by this Glassfish Eclipse plugin). All happens in a subsecond while Tomcat usually takes 3 seconds for this (yes, Tomcat is then basically restarting itself).
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