Your question can be explained in simple words- IISc is basically for science and research whereas IIM is for management. If you want to make a career in science you should go for iisc whereas if you are looking for a white collar job you should go for iim. ALthough iisc provides for M.Management but still it is not the first choice for some. I will give you the information I have from those who have studied in IISc- There is 100% placement in management studies with average ctc 16 lakh per annum. Professors are generally PhD. holders from IITs or IIMs with the fees being around 8 times lower than the iims.