If I were you, I would be going to McKinsey no questions asked. Even though it may be a lower level position, you will be exposed to so much and learn a great amount.
If you enjoy what you are doing, fantastic, you will likely be able to go back to school on the company's dime. In addition, that pre-mba experience with the firm will give you a big advantage against new Associates in your post-mba role.
In contrast, you could also decide after two years that you do not necessarily love the consulting lifestyle. Now, you have a better idea of what industry you would like to be in and what function you would like to be doing. Your McKinsey experience will help you get into a top tier MBA (M7, LBS, INSEAD, etc.) most likely with with $$$!
I really see no downside of taking the role at McKinsey other holding off on bschool for a couple years. Who is to say you will even need to go back to school at that point to pivot. Just my $0.02!