American Universities are much harder
ocean11 is right...you crazy. Believe me, American universities are NOT harder than Canadian universities. I'm in 3rd year at U Toronto, studied my little @$$ off wrote the MCAT last August and, well, let's just say I did better than I ever expected. If you go to a Canadian university -- especially one of the big ones (Dal, UofT, McGill, Queen's, UWO, UBC, etc.), I think you're in good hands. That said, performance on the actual MCAT is more down to the individual and his or her own preparation rather than the school.
If there is any section of the MCAT that Americans have an advantage on, it's the writing sample. Unfortunately, the prompts tend to be very pro-American and ethnocentric. Many of them either allude to American history that we in Canada may not be familiar with, or demand that we regurgitate patriotic crap about American democracy, civil rights, moral uprightness, and other such myths. I writing sample is easily conquered with practice, but be aware of who wrote the prompts, who is marking them (Americans), and what types of answers are expected.