I had to make a similar decision not too long ago - I understand how tricky the answer is.
What I did is I answered the following questions:
1) For how long do you want to stay in consulting? (if you are planning to stay for another 3+ years, then maybe the change makes sense because by then you could be a PM at McK which could offer you much more attractive exit options than being a PM at Big4)
2) Would it be ok for your to lose the project leadership role? Is it ok for you to "demote" yourself from a manager having full delivery responsibility back to the associate level? (if you can see this transition as an opportunity to fine-tune your toolkit and you can accept not being the captain of the ship for another 2 years, then the change makes sense)
3) Do you still have the energy to go the extra mile to prove yourself and rebuild your office brand in a new environment?
I have friends who made the switch not too long ago and they were both very happy with it - so I decided to do the same. Therefore, it wouldn't hurt if you ask some people from that office to see what they have to say.
I hope that helps.