Comparing Modules

I don't see such a feature either. Seeing the difference between a collection and a module, I would say it will require a lot of effort to implement such feature for modules.
https://jazz.net/help-dev/clm/topic/com.ibm.rational.rrm.help.doc/topics/r_compare_collections_modules.html
Notice that when comparing collections, it only reports the difference of "inclusion" of artifacts. On the other hand, a module is a "structured" artifact, and you will expect to see difference of the structure (order and hierarchy) when comparing two modules. It will be quite a challenge just thinking about how to report the "difference".