My uncle is an alumnus of IIM Calcutta and also an ex-employee of ISB. So I asked him about the same. He told me that the faculty of IIM Calcutta focuses on the overall development of students and not just academics. They prepare you for the life ahead and not just for exams or placement. ISB on the other hand due to its short duration course is unable to cope with this aspect. Also, the fees of ISB are much more than that of IIMs. The only merit is that it saves you a year as the flagship program at IIMs is two years.

So ISB is a good choice for you only if

• You are looking for a course of shorter duration 
• You are okay with spending a lot of money
• You value the degree more than what you learn from it

Both the institutes are world-class and therefore not much comparison can be drawn from them.