Here are links to the wikipedia articles for the two:
Apparently Montreal's is longer, although part of that length may be above ground (but connected to the system). Toronto's is the world's largest underground shopping complex, whereas Montreal's includes a lot of non-shopping areas, like blank tunnels, train stations, etc. . Toronto's was started earlier, although didn't really get going until the 1970s. Montreal's started being made in the 1960s.
I seriously still think Montreal's is better known, though!
And apparently there are some other, though smaller, underground networks in Canada.