mcnabakuk:
If you are applying for R1, you are probably better off staying where you are at MBB. If you are applying for R2, it becomes a little more mixed. The reason being, you will not be able to accumulate tons of experience at this startup company. Three months is not going to be considered long enough to have reasonably made a huge impact or formed the sort of relationships necessary to move the needle for the schools you are targeting.
For the record, you do not actually lose the MBB name. It's still an important part of your application from a time perspective. I also have to imagine you will seek recommenders from your firm. The start-up would be valuable because you are going to amass 12 months of experience, give or take, by the time you would matriculate.
How much do you focus on tech now? If you felt the startup was in tech and could be a great opportunity to make a stronger case for your post-MBA career, you should consider it. Again, only for R2 in that case.
For what it's worth, I don't think the prestige of a startup versus MBB is really at play it when it comes to admissions at these schools. It's really all about how you sell your responsibilities and accomplishments.
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