I queried Google Maps ... it showed three routes ... I-94, I-90, and I-80
I have not travelled the I-94 route extensively.
I have travelled all of the I-90 route from Casper, WY to the Dells. It is mostly four lane road (some two lane between Douglas and the Black Hills but the lanes are wide with nice shoulders) ... from Rapid City this route is as flat as a pancake.
I have travelled the I-80 route from Salt Lake City to the Dells ... there is a good uphill pull just east of Salt Lake City (you will avoid this if you take I-84) and another significant pull just east of Laramie. There are lots of trucks on I-80 between the junction with I-76 in western Nebraska and where you would turn near Iowa City, IA. This route has the lowest elevation to cross the Rockies .... just under 9000 feet.
The difference in distance and travel times is not significant ...
2004 Winnebago Journey 39W - 2012 Buick Enclave
Present @home Home: Oshkosh, WI
We call our rig "Ernie the Journey"