Integration testing is done at module level when various modules are integrated with each other to form a system or sub-system. Its main purpose is to ensure that interfaces between various modules are working properly; i.e. modules which are working individually are also working correctly together.
System testing is done on a complete, integrated system to evaluate the system's compliance with its specified requirements. It validates that the system meets its functional and non-functional requirements.
From these definitions, it is clear that the purpose of Integration and purpose of System Testing are different. Therefore, Integration test cases focus more on the interfaces between modules (interface integrity) - the data transfer and their interaction with each other. System test cases focus on testing the product as a whole; i.e. whether the functional, non-functional requirements of the System are met or not. Since System Testing is the final phase before delivery of the product, System test cases should pinpoint configuration related errors along with testing for performance, security, reliability etc.
