Congratulations on your admit. It's great to have options. Although i am not an expert, but here goes what I think.
IIM A : Great school, great brand name, awesome opportunities, small class which is a great thing, and a class that is more or less experienced in the range you are. Thus, you will have more peer learning, a very important aspect of any MBA course, compared to ISB, which has reletively younger class. Although PGP is the flagship course at IIM A, PGPX cannot be undermined as the faculty and resources at your disposal remain the same. It's upto you, how you decide to take things further. However, I would advice you to do your own research. Talk to as many current students as you can. That would help.
ISB: Again an awesome brand name, a great launch pad for your career enhancement, an unmatched global faculty. The only downside that I see, is the class strength. 800+ is a big number, and that is what scares me of ISB. I can imagine scenes at placement, no matter how efficient the department is.
Final words : Think hard, as to what you want to gain out of your time, money, and efforts. You will find your answers. If I were you, I would be spending my day talking to as many students and alumni and get all my doubts cleared. But I feel IIM A might just suit you more.
BR.