If you go the north route, you'll want to check out the campground availability and prices in western North Dakota (Dickinson Area) prior to going. A lot of oil activity in that region and not much availability from the last time I checked 3 months ago and the few that were available were pricey. There will be a lot of "permanent" setups due to the lack of housing for the oil workers in campgrounds.
We stayed in Jamestown, ND at Jamestown Campground, it wasn't very full with long term people. Reasonable prices for overnights. It's a nice park, the county gravel road to the campground needed a little more work, but passable and not very rough.
I-94 is very nice scenery through the badlands area of North Dakota (over by Montana). Would plan a stop at Roosevelt National Park to do some more touring if we had time. It's similar to SD Badlands but not as touristy.
The 2 lane diagonal (US 87, MT 3, US 89, US 12) from Billings to St. Mary (Glacier NP), is scenic and not very steep, it's mainly high plains desert/prairie. We drove that route a week ago, it's all nice 2 lane with good passing opportunities if needed. Couple real good stops for fuel/break throughout.
One idea if you have the time, is to take one route (IE I-90) out and the other back (I-94) since the roads both join again at Billings.
