Either will work - US$$$ are accepted most everywhere, but merchants may not recognize the true value of the US$ vs the C$. If you prefer using Canadian $$$'s you can get it at ATM machines, but the downside could be that any surplus C$ won't be easy to use in Alaska.
Personally, I would use a credit card and carry US$ to cover off where your card may not be usable. I use AMEX throughout Canada and the US, but because their fees are higher than other cards, some businesses, service stations etc., do not accept AMEX.
Alaska travellers always appreciate using the travel book 'The Milepost'!
http://www.themilepost.com
PIGGI, I was through Sahuarita on Friday - took a ride to Nogales and back - if I remember correctly temperatures were pretty close to 100F in the area! A great ride but certainly needed to stay hydrated!