Taking the diagonal from ND to Edmonton is a good route. Just beware that Williston in western ND is the center of the oil boom, with attendant truck traffic and overflowing temporary accommodations.
Some argue against crossing into Canada 'too soon' because gas prices are higher there. But, that's just one fill up out of many.
From the Black Hills, the more western route, using I15, is about the same distance as the northerly ones, and may be faster, since it uses more freeway.
Do consider visiting the Canadian Rockies parks if you haven't been there before. But that can be seen on the way back just as well. From Jasper you don't have to go all the way back east to Edmonton.