Difference between system testing and system integration testing

System integration testing:   individual modules are combined and tested together
System testing: testing the system as a whole.

System integration testing:   testing the interface between modules, it can be top-down, bottom up or big bang approach depending on development process.
System testing: it tests out end-to-end business scenario.

System integration testing:  it can be tested multiple times when one modules is complete.
System testing:  it is conducted in the final stage after integration has completed. 

System integration testing:   low level testing
System testing:  high level testing