I've lived in Montreal (born & raised), Silicon Valley, and currently Vancouver.
You're right that in Montreal, investment dollars will go much further, and it is a more interesting city for a young person. If you are looking for a place to incubate, as a young company, Montreal might be almost perfect. Similarly, it's a great deal for a student.
But then you have to grow up and live there.
The tech talent pool is very thin, the laws are mindbogglingly perverse, and the taxes are very high. The French/English split becomes very difficult when you think about bringing more workers over from a non-francophone country, not to mention their spouses and partners.
Just like foreign students, a YC company that's growing up will probably want to get the hell out of Montreal.
At least in Vancouver you have some kind of chance to hang around, and if you want more techies there's a reasonable local talent pool, and people from the rest of the world don't mind moving here so much.