Hello and welcome to the RTA forums !

The reason you are getting 'conflicting' results is because there is rarely one 'best' route, whether that be the quickest or the most scenic, or a combination of both. Interstates are built with gradual curves and gradients so you shouldn't have a whole lot of problem driving at elevation. If it were I and I wanted to get there quickly with the best 'bang for the bucks' [scenery] I would head up to Flagstaff, Moab to I70 to Denver and then I76 to 180. You could take I40 to I25 to Denver which is similar time and a lot of other route combinations are available. How much time have you got ? Either way it would be a minimun 3 day drive to Minneapolis, but Omaha would be beyond what we would recommend travelling in 2 days, so if you plan to overnight in Omaha, you should allow at least one extra day.