AFAIK, eZContentObject::clearCache() is usually enough to circumvent memory issues in CLI scripts because it clears a cache that stores all content objects in memory while eZContentObject::resetData() only clears the data map of the object that can be quite small and should be cleared by clearCache() anyway.
I already had memory issues using only eZContentObject::clearCache(). Normally it would indeed clear all the circular references to the object, it actually doesn't. I don't know why, maybe a PHP issue but anyway, the best way to avoid memory leaks is to use both eZContentObject::clearCache() and eZContentObject::resetDataMap().
Here you can find an example in SQLIImport content framework (loot at the flush() method, call by the destructor) :
