On the choice of the city, it all depends on what those cities can offer. I can't say I'm terribly familiar with either of the choices you presented, but see what kind of High Concept aspect you could apply to either of them and see which you like more. Just as Chicago is called the crossroads of the world in the novels, so too should the city you choose have its own tagline and city sheet that is provided in the rules. In the Dresden game I'm in at the moment, we're in London and calling it the melting pot, which in turn means that we can find all sorts of people and creatures in the city. You'll definitely want to emphasize on the city a lot, find what kind of things you can put into it.

So, I suppose the next question is: why either Boston or Montreal? What are the two cities offering that interests your group, and what are the pros and cons of each?