You can get into MBB (and most other consulting firms) without any business degree. For many years, the head of McK Germany was a theologist, I believe. And you already have a business degree, which definitely will not hurt. And both schools are excellent and well-renowned, so that helps as well.
So by all means, go for the Master that fits your personal interest! It's probably the last time for a long time where you are totally free to do what tickles your fancy and not to do what someone else expects from you. So go do it!
Cheers & good luck,
PS: My personal opinion - if you would pursue a degree just because you think it will increase your chances of landing a job of which you think you will enjoy (but don't know), but your heart is not in it, you will be miserable and probably regret it for the rest of your life.