Well, I think I could help. I have lived in all three cities mentioned, so perhaps I could divulge a little more perspective.
I have spent most of my life in Vancouver, where I am currently situated. The winters here are much more mild than everywhere else in Canada. Vancouver is on the edge of the largest temperate rainforest in the world, so we have LOTS of rain all year-round. That was the majority of our winter this year. However, with that climate, the cold here can still 'feel' really cold, even though it is less extreme than elsewhere in Canada. Vancouver is also the most expensive city to live in across Canada. The wealth here is extreme, but so is the poverty. It's beautiful, but also deceiving. Winter people have the mountains, summer people have the ocean. The diversity among people largely depends upon where you are located. Within the city, it's fairly diverse and multicultural and Vancouver has an enormous Asian population (Hongcouver, Japanada). However, city-living is very expensive, and cheap places are hard to come by, unless you are willing to live in East Hastings. As for gay clubs, we have Davie Street here, I have only really been to Celebrities and the Odyssey, which are both fun (mostly), but Celebrities is pretty mainstream and has a lot of straight nights; the Odyssey is just plain gay, but it has more entertaining people (naturally).
Lived here for three years and visit often. Biggest city in Canada and also the most multicultural city in the world. This means that there is a lot of good places to eat everywhere, there is a Polish area, an Indian area, China town is great (Kensington Market), and literally almost every culture you can imagine, and you get all that food at your fingertips. Amazing. Transit system is good (unlike Vancouver, which is annoying if you live outside the city). It's a big city, so there is always something going on. Winters can be rough, cold and snowy, but you can bundle up and survive, whereas in Vancouver it's more of a 'wet cold', so it chills you to the bone. But it does get COLD in Toronto. And it gets hot and humid in the summer. The gay street is Church Street, and it's kind of trashy, not gonna lie. I don't know the good clubs there, but it's Toronto, there will be something. Toronto is expensive depending upon where you live, but not as bad as Vancouver.
Spent my last two summers in Montreal, and will be moving there shortly (as soon as I am off crutches). For the record, I am bias in my assessments because I am moving to Montreal, so I obviously think it is the best city, but not everyone agrees. It is extremely multi-cultural, not as varied as Toronto, but much more-so than Vancouver. It is the MOST gay-friendly city in Canada, and could be competition for San Francisco. The gay cubs are abundant, some cheap and trashy, others fun and the best is the six-story lesbian bar - Drug Store. ha! It's also the cheapest of the three cities, and I can afford to live in a really good neighbourhood for the equivalent of what would get me a small room in a shitty apartment in a street with more prostitutes than pedestrians in Vancouver. Food is much cheaper here too. Montreal is a bilingual city, so it is French-speaking and English-speaking. You can get by with one or the other, but if you're going to be there, you may as well learn French (or try to), it WILL get you better service. Also, it helps to make friends with Anglos and Francophones, and then you can have friends who will be HAPPY to teach you French. You will also be exposed to poutine: fries served with cheese curds and gravy. It's amazing, but only eat it while intoxicated. It should NOT be a regular part of your diet. Montreal also has hot and humid summers, more so than Toronto, and their winters are much worse than Toronto's too, tons of snow and it can get REALLY cold. It's a big party city, and in the summer there is ALWAYS something going on in the city, which leaves the main street - Saint Catherine street - pretty much closed for pedestrian traffic and music festivals all summer long.
Here it is simplified for Americans.
Vancouver = Los Angeles
Toronto = New York
Montreal = New Orleans (pre-Katrina).
Hope that helps.