Hello Swati. IITs and IISc are two very different type of institutions. IITs are more of institutions of professional courses. It focuses on mainly engineering studies whereas IISc focuses on pure sciences. And both are premier institutes in their respective fields. However When we see the performance of the institution on a whole, IISc rank far better than any other institutions in India in prestigious Rankings.
IITs were established to provide finest technical manpower for building the nation. IISc was established by private bodies to conduct core research with focus on advancing the scientific capabilities of the country. As IISc have finest faculty, creators of a field of science, they have contacts all across the globe. Their labs are so famous that the students can easily get accepted in any of the top university in any nation. Officially IISc have many research collaboration with industry and foreign universities that IITs are in their infancy now a days.