The airport money changers are often the worst place to convert. Usually, debit cards get a better rate, although they vary by the Canadian bank. Years ago (been enjoying Canada for about 15 years,) must all debit cards at ATMs gave you the real latest exchange rate. But the last few trips tried various banks ATM's and some were at the real rates and other marked up (I forget which bank I found best). In the US I have a Bank of America Debit card I only use for travel to Canada since they have an affiliation with Sociabank and no fees.
The which currency to use is obvious in Canada since with our current strong $USD buys about 15% more $CDN and of course, folks in Canada don't usually take $US for obvious reasons - you're in Canada.
I am doing a lot of research on options in Tijuana Mexico, Lots of escorts, the strip clubs - forget the street ladies - but am considering in early November, instead of Toronto since its getting too cold up there! Went to Germany instead of Toronto for my usual June trip.
What is odd in TJ it is more common to take and want $US even in Mexico - at least in the Adult Zone near the border. Escorts are typical $80-125 an hour all outcall to your hotel but want $US. Financially it would be better for us to pay in Mx since the US dollar is so much stronger.