Congratulations on two great offer.
There's a short term view you can take and a longer term view. In the short term, the higher salary seems more enticing and Kearney does that on purpose because they know their brand name cannot compete with MBB. However, in the longer term, MBB folks end up making similar or more money because as you rise through the firm your pay increases very quickly as well.
The other angle obviously is the prestige factor. When you exit from Bain, there is a certain extra you can negotiate from a salary perspective in a new job given the reputation of Bain. This premium is smaller for Kearney.
Despite all of this I think you should go where you feel happiest. If Kearney makes you happy (based on the people, the salary and culture) go for it. No one can predict the future anyway and just because Bain is a bigger brand does not always translate to a happier career.
(editiert)