Well, code organization (and hence maintainability) is the first thing that I can think of (assuming you are not using any service provided by the Java EE container).
If your system is going to be small and simple and you are the only (or one of the few) developer, then probably you will have your system (or portions of it) done faster in one of those scripting languages (if you are already comfortable with one of them).
However, if your system will get bigger and more complex, or you will have more developers to work with, then Servlets will probably be a better choice.
Since you already have a Java EE container running (for your JSPs), using scripting languages will not be more lightweight in runtime (unless you re-code your system).
Just another thing, maybe you get more / better answers in the Servlets forum
[ July 23, 2008: Message edited by: Rodrigo Tomita ]