The two civilisation laid their foundation on river valleys. Mesopotamian flourished on banks of Tigris and Euphrates and the Harappan civilisation made its foundation on banks of Indus valley. Today Mesopotamian civilisation is known as Kuwait and Iraq and one of the most important city war Ur and presently Harappa civilisation Pakistan, part of Afghanistan and Northern India. Important cities are Mohenjo-Daro. But Mesopotamian and harppan economies developed along the similar bases, and having comparable religious and social structures. Yet, their politics, arts and women treatment have a sharp contrast to each other.
The both civilisation are politically different. Where Mesopotamians developed the first monarchy, and harppans to develop the first democracy, very less evidence has been discovered of a king in Indus valley, only one white priest-king and a silver crown was found which is not enough to say that there was royal rule in Indus valley. Instead the empire was divided into regions with half a dozen cities functioning as capitals and was governed by a group of people. Archaeologist Jonathan Mark Kenoyed has speculated that the Harappan rulers were merchants, ritual specialists and individuals controlling important resources, instead of just one social group controlling the rest. From the construction of the cities however it does appear there were some social classes, as the citadel is usually 20 feet higher than the middle and lower town.
The Sumerians, on the other hand, developed into city-states ruled by a priest-king. The king led the military, administered trade, judged disputes and performed certain important religious ceremonies. The king also had a bureaucracy, which consisted mostly of priests, who assisted him in governing. To justify the authority of the priest-kings the Sumerians declared that the king was divinely selected, but later this changed and eventually they asserted that the king himself was divine and worthy of worship. So while the Sumerians worshiped their king the Harappans chose not to glorify any particular person and were instead ruled by ordinary people. Both the civilisations are very similar. Both relied heavily on trade; infact the extensively traded with one another.
In many evidences found in Mesopotamia there is a mention of a civilisation they use to trade somewhere in Indus valley and many famous seals have been found at Mesopotamia and in return they exported textiles and crops, while harappans traded lumber, copper, gold and ivory. Both, civilisation were well advance in transportation services and they both used different types of sail boats to travel across. In terms of religion, we know that Mesopotamians religion and even less about harappan, but we do know both were polytheistic. The Sumerians (a civilisation within Mesopotamia) believed in many human like gods with also human emotions.
They believed that sun, moon and stars were gods and everything happened by their wish and we created to serve them and our destinies are controlled by them. By, doing this they discovered astrology. The lesser gods, who did not have enormous temples built for them, were idols and statues, which were worshiped in homes. Since, the Indus script is not deciphered and apart from bath house in Mohenjo-Daro and there no religious structure found. What we know is from figurines and seals, many of which depict a horned goddess with the sacred papal tree. This probably indicates the worship of a mother goddess who presided over fertility.