I have some things for you to consider in making your choice, in no particular order:
McGill focuses very much on the Montreal market so if you're sure this is where you want to start your legal career, McGill is the place for you. You will be able to go to networking events and the CDO will have more comprehensive information for you.
McGill also places some students in Toronto (this year it was ~20 students) and New York (a handful), so if you're interested in those markets, perhaps McGill is a better choice over UBC. But, you will still have to work to find information about those markets. McGill does place very few students in Vancouver, but you should expect to be proactive in finding out information and seeking out networking events. These students are usually from Vancouver too. So if you want to return to Vancouver, it won't be a problem if you're proactive about looking for summer and articling opportunities in Vancouver with help from the CDO.
What makes you enjoy Montreal over Vancouver? If it's based on short visits to Montreal, I would say that is very different than networking and living in the city for a couple months. After staying awhile, you might find you don't like the Montreal culture or the legal culture there. Also, make sure you sort out insurance and who will be your health providers in Quebec, it can be a pain if you have chronic health problems or just a hassle in general to be out-of-province.
If you end up coming to McGill and decide you want to do Montreal recruitment, make sure you are ready to explain "why Montreal" because employers want to make sure you'll stick around. That is not to say students who have no ties to Montreal aren't successful, there are quite a few students who decide they love Montreal and want to work there with no prior ties.
Knowledge of French won't be an issue at McGill. I don't think you need to work on it further to make it through the program. However, it sounds like your French is conversationally decent and if you want to work in Montreal, you will likely have to actively work on it so that you can speak it in a professional setting. This includes being able to interview in French. It might be easy to plan to work on your French now, but you will have to be committed once you start first year because it'll be easy to push it off to the side.
You will also have to go to bar school and pass the bar, which will be a gruesome 4 months (full time) or 8 months (part time). Since McGill is "transsystemic," it doesn't do a good job of preparing students for the Quebec bar, unlike other schools. Students don't really use the civil code like other Quebec law students.
The McGill program is structured for 3.5 years (students may graduate in 3 years if they obtain the required credits in time), which will throw some employers off. Going forward, this is something to consider depending on which market you want to work in. It may be an annoyance and also a barrier to employment to explain it to employers who are not familiar with the program.
Hope this helps!