Like most MBA answers, it depends.
1. If you are a fresher go to IIM. If you have 3-4+ years of experience go to ISB.
2. If you want a finance job go to IIM. IIM lobby in finance is very strong.
3. If you want a consulting job go to ISB. All consulting compaies come to ISB.
4. If you want to experience a foreign MBA in India go to ISB. Most of the faculty are visiting from LBS, Harvard, Kellogg and they are the best I have ever seen. Don’t know about IIM.
5. The students mix is also more international, with many of the returning Indians from abroad use ISB to get into Indian job market.
6. ISB is 1 year, IIM is 2. It is an important distinction.
7. ISB has established a solid reputation in just 10 years. Still the management, staff, faculty, students and alumni know that lot more needs to be done and are very proactive and enthusiastic about improving the college. I am sure the same would be at IIM.
8. It is tough to completely change your domain of work from ISB, as very few entry level jobs come on campus. Nobody will make you a Brand manager for a FMCG product if for the last 5 years you were in IT and majored in marketing. IIMs get more entry level positions so domain change is easier.