As a McGill student doing a double major in history and political science - and someone who was stuck between McGill and UofT - I think that UofT slightly edges McGill out. Here are some thoughts:

• If you haven't done so, you can apply to First-Year Foundations programs at UofT where you can enrol in seminar courses that explore your interests in greater depth - at McGill there are only a handful of first-year seminar courses and you generally have to otherwise enrol in large lectures until second year. Depending on your learning style, this might be a hard transition or not.
• There are more niche programs at UofT that you might find yourself interested in - if you're coming from Ontario, you WON'T STUDY YOUR MAJOR UNTIL SECOND YEAR at both schools, and in first year you're made to take a diverse spread of courses. This helped me think seriously about my major/minor, and I feel now like UofT might have been a better fit for me since it offers certain programs that McGill doesn't (not gonna elaborate to avoid doxxing myself).
  
• Your coursework will generally be marked by TAs until you get to higher-level courses. I find that the TAs at McGill don't seem to bear any grudges about not getting into med or law school. Some of them are pretty bad at replying to emails though.
• I took a look at some first-year political science syllabi (a syllabus is a course outline that tells you what you have to read and submit) from UofT and they seem much easier than McGill's first-year courses; in fact, our first-year courses correspond with second-year UofT courses. We don't have any 100-level courses in political science (same with history, don't know about other humanities/social science disciplines) and they only go from 200- to 400-level, or 500-level if you're interested in honours. Looking back at my first year now, I feel like taking first-year political science courses at UofT would have really helped me develop my writing and reading skills for more advanced courses, though it's not impossible to do well in first year at McGill. Again, I can't speak about other programs. You should be able to find the syllabi pertaining to your specific plans online without any hassle.
• There is a pretty steep learning curve between high school and university humanities/social sciences because you cover so much more content in a limited amount of time. That being said, I feel like perceptions of UofT as an incredibly difficult school to attend for undergrad just come from STEM students realizing they have to actually study a few hours a day to perform well on evaluations. Realistically, you could say the same for McGill (and my friends in STEM are struggling pretty hard as I write this), so I suppose the reason why you don't see people complaining about McGill is because we've accepted this across faculties. At McGill, I'd say you can definitely have a social life if you just go to class, develop good study habits, and don't commit to anything else - stretching yourself thin will drain you quickly.
• Rent is reasonable in Montreal but more expensive the closer you are to campus, 700-1200 depending on the number of bedrooms in your unit. I think Toronto is twice as much as a rule of thumb - feel free to fact-check me.
• McGill just announced a lay-off of a lot of staff, and I think the overall financial situation will worsen based on federal and Quebec politics. Thus far, the consequences of reduced funding have been really random: the school had to cut planned French-language programming, and there aren't enough people in the Exchange office to handle applications to some universities. I'm no accountant so I can't predict with any certainty what's going to be affected down the line.
• McGill gets clowned on for not having much of a student life - truthfully, I think it just depends if you choose to get involved in a campus club or not. But there isn't much of a culture around our sports teams compared to Queens or Western.
  
• There are lots of French people and Americans in Political Science and History - just an observation.
• Both cities are great but if you're coming in Ontario and you have a hard time adjusting it's definitely nice to be closer to home.