generally speaking, the difference between Bain and McK and BCG is country-dependent. In many countries, Bain is still the smallest in the MBB group, thus also the one where, with a smaller office, there is also a more entrepreneurial/family-like environment. However, this is not the case in every country (in Italy, for example, Bain entered through the joint venture with another firm, making the company quite similar to McK and BCG in size from the beginning). As a consequence, I agree with Guennael that country differences could be more relevant than differences between firms.
Referring to the question "Why Bain?", to properly prepare such question, it is crucial to understand why companies ask it.
Of course, every consulting company knows that 90% of the interviewees are doing multiple application in consulting, and also that you may prefer another MBB company. Thus why such question?
The key reason is that they want to see if you are able to do due diligence in advance on your target/prospect. If you are going to an interview and have not studied the company you are going to interview with, chances are you are going to do the exact same thing with the client. And a consulting company definitely doesn’t want this to happen.
Thus the question translates in how you can show you have done due diligence on the company. A general structure to identify elements that distinguish a company is the following:
- Company potential: how much the company grew in the last year, which is a good guarantee for your future career as well
- Sectors of interest: specific focus of the company in areas interesting for you
- People you met and culture: specific people of the company you discussed with, and the things they shared on the company
- Your growth opportunities: specific programs of the company you heard about to enhance your growth
- Size/reputation of the firm: smaller companies usually offer a more entrepreneurial culture, with higher growth potential internally. Bigger companies have more resources and support staff, and more prestige in terms of exit.
Whatever topic you choose, you can apply a simple rule of thumb: if you can substitute the name Bain with another one, and have the answer still making sense, it means your answer is not specific enough. To be specific, you should talk about growth rates/specific people you talked to/specific research paper you read on the company.
This means you will have to spend some time preparing this question. But it also means you will have higher chances to stand out - very few candidates invest time in such due diligence.
Hope this helps,