My favorite planning website is google maps. But i use that alongside
www.roadsideamerica.com (Canada provinces are included) so i can see if there are any fun or quirky sites or things to see along whichever route i am taking. It really makes the route fun.
As for whether you should go south or north of the border depends on what you want to see. Once you are in the middle of the continent, ie midwest US and Manitoba/Saskatchewan, it is mostly plains and flatland with lots of grain fields (they call it the bread basket for a reason), and scenery to match. If it were me i would stay on the CA side and take the Canadian Transcontinental Hwy 1 all the way to BC just for the history of the road and cooler northerly temps.
Or if you have never ridden through Michigan and the upper penninsula, that is supposed to be a great area to ride, plus you could hit the Mackinac Bridge. Then on to Winnipeg and onto Hwy 1.
Whichever route you end up will not be wrong since you will be riding.