From my limited experience of working with McKinsey people, I would describe it as a Fortune 500 C-Suite incubator. The hire smart people, they are very well connected and the goal is to rise the ranks and get a CEO position in a multinational. I was told by a very senior ex-McKinsey consultant that their main role is to get people promoted (regional CEO to central board for example). The projects are essentially a vehicle for an individual to move up. Their network is phenomenal. I wasn't very impressed with what i saw productivity wise, but they are polished and the point is not to impress the lower downs.
Accenture are mostly a bunch of low level talentless grunts. There are a lot of them. If you're hiring Accenture then you are in trouble in more ways than one.