Hi there,
Yes, the office matters. Coming from Australia BCG I can assuredly say that I'm glad I was there and not in, say, Germany. Work-life balances and prioritizations are pretty predictably tied to country cultures.
Furthermore, where you work changes what industries/function you work in, who your network becomes, etc.
Think about what you're optimizing, but remember there is no "best" spot. US/Germany may train you best but be utterly exhausting. Australia may give you better worklife balance (still 60-70 hr/weeks) but give worse training (it doesn't, but this is an example). Neither will help you build your after-BCG network in Spain, which is super important for success.
Moreover, you're assuming you have the luxury to pick. You kind of don't...apply where your resume actually makes sense. I highly advise you not to play this "game" of offices, and just network+apply in the office where you having working rights, education, work experience, etc.
I am starting in the Miami office (got accepted) but I eventually am considering Italy (where I've been living for a bit) or Spain (I speak both decent Italian and fluent Spanish). I was eventually considering a long term life in Southern Europe.