Hi Barry
Personally I would use Canadian dollars in Canada - you can withdraw some easily from ATMs but remember to let your bank know about your travel plans if you do.
I understand you can use $US to some extent, especially when you are close to the border - Niagara for example, as it is visited by a lot of US tourists. But you will find it harder to use them the further north you go. You will also find you are charged a variable exchange rate by the shops that do accept $US.
Kind regards
Olly, your editor