In terms of job opportunities, IIM is far superior.
IIM A, B, and C would have an average placement of around 22 LPA, whereas ISI would have 16-17 LPA. However, you must keep in mind that you would pay at least INR 20 lac for your IIM degree while receiving a 65k stipend and contingency grant from ISI for free.
You might start out as a data scientist, financial analyst, or consultant if you join ISI, and you would advance along that career vertical. IIM exposes you to a much broader range of roles such as FMCG, general management, and so on.
In terms of higher education, 40-50% of ISI batches pursue Ph.D. programs in the US/abroad, while nearly 100% of IIM batches pursue jobs. Going straight to a Ph.D. after IIM is difficult due to the large educational loan (assuming you take one). However, many of them pursue Ph.D. degrees at various business schools around the world.
However, these institutes (ISI and IIM) are incomparable because one teaches statistics, and the other management. These are two very different topics. So it all comes down to what you want to do with your life.