I'm in pretty much exactly the same position as you (Columbia MA and Cambridge MPhil offers), so can't really give much advice other than that I have the same dilemma! I'm leaning towards Columbia because I'd like to pursue a PhD in the US and think it sets me up better for that, but think it might make more sense to wait a year after the MA before applying (i.e. not apply this summer).
One question I have is that I thought Fulbright meant that you had to return to your home country straight after you complete your course because of the two year home residency requirement. Does that jeopardise your chances of applying for a PhD immediately? I may be completely mistaken, though! If it does, however, that may play into the calculation if it makes it hard to go straight into the PhD.
I don't know about the reputation of IHEID Geneva; however, I think that you are right that quant is very important in the US. The option of studying abroad is also fantastic because it means you can get all the contacts with the professors in the US that you might need (and this does seem to be very important for PhD programs). IHEID Geneva may end up being the best of both worlds for those two reasons, but that's just from an outsider's perspective.