There are several things:
1. Reputation & (perceived) Thought-Leadership McKinsey is still the gold standard in terms of recognition across all industries, from tech to large established corporates. Whilst on some specific areas other firms might have developed a stronger expertise (e.g. Parthenon is the leading firm for education), everybody recognizes that hiring McKinsey is the safest bet to ensure consistent, high quality work. The level of knowledge created by McKinsey grants it top pages in leading newspapers, a coverage that is still not matched by BCG or Bain. BCG has been trying quite successfully to go in the same direction, establishing reputation as a knowledge-based company, but is not yet there. Bain, on the other hand, has been focusing less on knowledge leadership and instead cultivating a reputation focused on its capability of making things happen by adopting a cooperative approach.
2. Alumni network Consulting is usually regarded as a springboard to a brilliant career. This is why the alumni network that you'll find when leaving should be considered among the key decision criteria. McKinsey has the largest workforce (8k consultants vs 4.5 BCG and 3k Bain), hence the largest alumni network. Moreover there is a strong cohesion and sense of belonging fostered whilst working at McKinsey that usually remains after leaving. You'll be surprised at how many people who were "counselled to leave" by McKinsey (basically advised to find another job elsewhere) turned themselves into key McKinsey clients after leaving. Alumni networks are weaker in BCG and Bain due to the lack of critical mass and a weaker sense of belonging. However, BCG has become increasingly active in engaging its alumni network, and my subjective impression (as an alum of both McK and BCG) is that they have managed closed some part of the gap to McKinsey.
3. People All MBB firms are obsessed about hiring the best and the brightest. The smartness of colleagues in all three MBB firms is absolutely amazing. Here I'd say that the main difference in the hiring criteria between McKinsey and BCG is in consistency and ease of being "shaped" according to the standards. While BCG is mostly focused on hiring bright people, McKinsey values a lot the alignment in terms of mindset and the ease of "formatting" the candidate according to the McKinsey standard. This includes speaking using a top down approach, keeping a friendly but assertive approach, and most of all being open to feedback. McKinsey puts a lot of emphasis on the three above characteristics and regards them as a key decision factor in the selection process.
4. Client mix This varies to a large extent by office and by country; as a general trend, Bain has higher mix of PE clients (Bain Capital, although now separated from Bain, was co-founded by several Bain partners), McKinsey is the leading consultancy for healthcare and governments, especially in developing countries.
5. Projects It has often been said that BCG focuses more on pure strategic projects, McKinsey on organizational projects. This is no longer the case. All three MBB firms have been trying to move downstream in order to provide end-to-end solutions to their clients going from devising strategies to testing and implementing them. Moreover, they are all massively investing to ride the massive wave of digitalization that is still building up.
6. a) Approach to clients McKinsey tends to have a more top-down approach and usually takes pride in challenging and often contradicting clients. They do cooperate with clients but tend to be quite assertive in pushing forward their solution. BCG is slightly more “cooperative”, putting a special emphasis on “delighting” clients. BCG consultants often spend a bit more time in order to build consensus in the organization by engaging the middle management, instead of obsessing over analysis (McKinsey way).
6. b) Approach to problems All MBB firms leverage a lot past work to provide answers including tested, off-the-shelf elements. However, BCG has cultivated a reputation for being looking at each problem with a fresh perspective and building innovative solutions. It is also something that consultants like to hear when interviewing candidates and asking them "Why BCG?"
7. Way of working/resources A point that is often neglected in answers to this kind of question is understanding how everyday work will change across firms. Whilst there are many similarities (4 days a week on client site, teamwork, challenging lifestyle), there are also two key differences:
- Staffing: McKinsey is more global in terms of staffing, meaning that you are more likely to end up staffed on the other side of the country but also that you'll have more chances to e.g. do a project in a developing country if you are interested in development or government work. Also, the project teams at McKinsey are mor internationally mixed – hence you might find team members from completely different parts of the world staffed on a project in South Africa for example. At BCG, it would be more homogeneous, e.g., the partner who covers the client will staff most of his team from either his home office or the client’s geography.
- Support resources: McKinsey wants its consultants, including Business Analysts, to focus solely on value-added activities. To ensure this, it developed large organizations aimed at supporting consultants, such as:
- Visual graphic centers to ensure consultants can save time by sketching powerpoint slides and have the quickly produced by visual graphic teams
- Research centers for performing desktop research for consultants
- Survey desks in India and Costa Rica to set up web-based surveys for clients
- Analytics centers to help consultants in complex xls macros etc.
Whilst Bain and BCG have both developed similar support resources, they still do not have the scale of McKinsey.