Honestly speaking, none of the IITs are at the level of foreign universities today. IIT is an excellent choice only if your prospects involve settling in India. Globally, IITs lack the necessary worth when compared to top US, UK, French, and Swiss universities.

In case you want to settle abroad then the kind of universities that will help you do so are MIT, Harvard, Yale, Oxford, Cambridge, and the Sorbonne only. Sadly, IIT is still not credible enough to help you settle abroad.

There are other reasons as well why IITs are considered inferior to top foreign universities like Oxford. At Oxford, you will find such a diverse peer group of geniuses that the learning experience will become excellent. There are many who study unique subjects like History, Latin, Greek, Modern Languages, English, Engineering, etc. When you end up befriending such geniuses, it adds to the overall experience. You basically get to spend a lot of time with intelligent people with unique interests and this helps you develop holistically.

This diversity is definitely lacking on the part of IITs and they have a long way to go.

There are two ways in which this answer can be answered, statistically and Realistically. 

Statistically speaking, the two colleges can be compared on the basis of their acceptance rates. For IIT, one needs to sit for JEE wherein around 500,000 appear for this exam. To make it into IIT Bombay, you need a rank of 2500 or below. Based on this the acceptance rate is around 0.5% of the total students who appeared for the exams.

On the other hand, Harvard School invites applications from around 10000 students and around 900 are selected out of them. Based on this, the acceptance rate is around 9% which is higher than IIT Bombay. So statistically, it is easier to get into HBS than IIT Bombay. It’s important to note that the above data is not specific and only estimates.

Rationally speaking, both colleges are miles apart, and comparing them is highly illogical.