I took a look last winter at national average prices for diesel and regular gasoline. Over the course of the previous 12 months, they averaged out pretty evenly.
However, in late January, in Regina, diesel got up to 15¢ more than regular gasoline. This summer it was pretty much even, but in the last month, the spread has already hit 5¢. So depending on fuel prices and driving conditions, diesel may simply not be a benefit.