Flattest route to Denver

I've driven 287 much of the way you are contemplating. Most of it in a car, but the last two years we have done 287 from I70 to Hwy 50 with the RV. One thing to note, there are a LOT of semi trucks that drive that route, and, at least in that area, not a lot of passing lanes. When we went in July, we had lots of semis pass us and pull in uncomfortably close to avoid oncoming traffic, and we also got stuck behind an over length truck with one of those windmill blades (easily 100' long) going 45-50 MPH. It was very difficult to get around him.

Given the choice, I would do I25.