What you need to do is get the cost of the tolls and compare it to the extra fuel cost. We were just in Alberta and it was about $1.09/liter. $1.09 x 3.785 (# of liters in a gallon) = $4.13 CAD/gal. At an exchange rate of about .76 the equivalent in USD per gallon is $3.14. Don't know what gas is across the rest of the country but here it's about $2.09 for regular. That's a big difference that will add up quickly.
Other factors to consider are ease of route and time. You'd also have to do two border crossings. There's going to be pros and cons of both-you just have to decide what is best.