I-90 is much more scenic, imho, especially once you hit the mountain time zone (Nebraska/South Dakota). The South Dakota Black Hills (Badlands Nat'l Monument, Mount Rushmore, Devil's Tower Nat'l Monument, etc., etc.) is along I-90, as well as the site of the Battle at the Little Bighorn. Yellowstone wouldn't be far out of the way. I-90 through northern Wyoming is much nicer than I-80 through southern Wyoming, and Montana and northern Idaho beat the heck out of the Utah and Nevada desert. Traffic is also much lighter on I-90.
