I've driven I80 basically from Colorado all the way through Iowa. I76 from I80 into Colorado is one of the roughest stretches of interstate I know of. In addition, I80 has TONS of semis. Can make it more difficult to pass.
My only experience is I70 from about Colby, KS to Colorado. Seems to be less traffic, less semis and a smoother ride. That would be my choice.
2012 RAM 3500 Laramie Longhorn DRW CC 4x4 Max Tow, Cummins HO, 60 gallon RDS aux fuel tank, Reese 18k Elite hitch
2003 Dodge Ram 3500 QC SB 4x4 Cummins HO NV5600 with Smarty JR, Jacobs EB (sold)
2002 Gulf Stream Sea Hawk 29FRB with Honda EV6010