25-35 is large, but it's not that large for a major Canadian university (and McGill is top-3). Compare to the University of Toronto, which has an MA cohort of around 40-50 students but the program is still ranked higher than McGill. In fact, the MAPSS cohort at Chicago is way larger--around 250 students, roughly 40-50 of whom do Poli Sci, so you'd be in a smaller program at McGill. Either are good options. If you get into both and the cost is roughly the same, you should be looking at which professors at which schools do work in an area that interests you.
I think you have a good shot at funding at Chicago. As long as you have decent GREs, I would guess you'd get something like 1/3 to 1/2 tuition reduction ($20,000 to $30,000). If you do really well on the GRE and your statement of purpose and other materials are on par, then you would have a chance at a full tuition reduction, or something close to it. I'd say that if you work hard at these things, it would be reasonable to expect a 1/2 tuition reduction. In that case, it would probably be more expensive than McGill (I'm not sure what international tuition at McGill is since I'm Canadian). Even if tuition were similar, the cost of living in Montreal is just a small fraction of the cost of living in Chicago. That being said, Chicago has faculty that are generally more well-known and influential than McGill's faculty, so it would be a tough call. Either way, if you get into either or both, you should be in good shape.
If you have any other questions about the universities, or the cities, let me know and I'd be happy to try to answer.
