This adapts a post I did on September 23, 2013 while travelling and writing trip reports.
Toronto has both older and newer silvery cars, small wheels. Montreal has newer looking blue cars, jumbo tires.
Toronto cars are only a little below ground and sometimes above ground. Montreal cars tend to be a lot further underground and to my knowledge never go above ground.
Toronto cars are a lot faster.
I believe Montreal covers a larger area, definitely with more stops. Toronto is entirely in Toronto while Montreal cars dip into Laval (3 stops) and Longeuil (1 stop). [Addition June 3, 2015: Toronto: some areas were different municipalities initially; Montreal: also one stop each for the enclaves of Westmount and Mont-Royal.]
Toronto has separate turnstiles for entering and exiting. Montreal uses the same turnstiles. Toronto turnstiles use tokens or card swiper, Montreal makes you tap a card.
Same number of lines if you count Scarborough’s above ground Scarborough line; in Montreal line names are colour coded. [Addition June 3, 2015: Toronto’s subway system is now number coded.]
Montreal has a definite hub that most lines go to. Toronto has no such hub.
Toronto has more exits from the rail area, though both allow multiple corridors when you get upstairs.