Hello IntlSocialWorker!
How much did U of M offer? As a matter of fact how much did they each offer? It ultimately depends on what you are looking to gain from the experience. Michigan has Detroit (historically impoverished city) to offer and they have the special programs like the Community Based Initiative Program, then Columbia as you mentioned has NYC as a potential field placement which is amazing, rich in culture, busy and dynamic and then you have Chicago a vibrant city that also has very diverse communities. Taking into account your interest in refugee/immigrant rights I would encourage you to look into the demographics of the cities, don't quote me but I believe Detroit and Chicago and predominantly Black communities and New York is more of a mix which I think would be very interesting for the work you want to pursue in terms of policy, immigrant rights etc.
Hope this helps!