The main difference between the systems of rule of the Neo-Assyrian and the Persian empires was the importance they put on centralized administration. The Assyrian system was much more centralized. Provincial governors answered directly to the king, who had ultimate authority, and to the royal court. The Assyrian governors also...
functioned as military commanders in the areas that they governed. They could punish and persecute the inhabitants as they saw fit.
Persian administration was based on the Assyrian model but was far more bureaucratic and humane. Each province had its own governor, or satrap, who reported to military representatives and to royal secretaries. Like under the Assyrians, conquered peoples were required to supply the central government with large amounts of tribute.
Both the Assyrians and the Persians had large multi-ethnic societies. This was the result of conquests that brought many different nations under their dominions. The Assyrians considered any of the conquered peoples to be slaves. These peoples were exploited for whatever purpose that the leadership saw fit. One way that the empire was integrated was through the forced relocation of conquered people. They were often relocated to the urban areas of the empire, exposing Assyrians to the customs, languages, and religions of the various peoples within the empire. Cultural integration also occurred through the process of intermarriage over the course of multiple generations.
The Persians also ruled over a large and culturally diverse area. However, they were much kinder to their conquered subjects than the Assyrians were. Many populations that had been taken captive by earlier regimes were released under Persian rule. The most famous case of this was the Jews who were released under the rule of Cyrus the Great. The Persians sought to impress the peoples under their dominion with the benefits of their culture rather than force this culture upon them like the Assyrians. Like the Assyrians, intermarriage also led to cultural integration throughout the empire.
It is unclear if the different approaches of the two empires led to more or less cultural integration than the other. Both civilizations were highly integrated but had a large degree of diversity as well. The Persians, though ruthless when necessary, were overall relatively kind and benevolent rulers. More people adopted the new culture willingly than those who had it foisted upon them upon them under the Assyrians. Persian traditions lasted well after the conquest of the empire by Alexander the Great. Assyrian culture did survive in some aspects after the end of the empire, but it was mostly supplanted by later cultures. Perhaps the gentler approach of the Persians led to a more long-lasting and resilient culture.