One cannot easily choose IIM Ahmedabad over elite business schools abroad like Harvard Business Schools. But if one actually does then the main reasons could be
Long term market: If you are willing to work in India for the long term, then IIM Ahmedabad is for you. This is because every business school provides best access to domestic markets. So if you want to stay in India IIM Ahmedabad offers the best access throughout Indian Markets.
Opportunities at fairly less cost: India is a developing country and its market is slowly coming at par with the US market. So if you are looking for an investment less earn more opportunity, IIM Ahmedabad is a good option.
Personal Convenience: It is not an exaggeration to say the quality of life you will be experiencing in your home country with your family will never be the same as that of abroad.
So both the B Schools are fantastic options. It depends on your convenience which to choose. If you wish to stay in India, IIM Ahmedabad is for you. But if you want to explore the US market go for any elite business school abroad.