Just returned from a run to Seattle and back.
Tried the I-90 route this time.
Observed that the road surfaces on this route are very, very nice. Except for the Colorado section they were mostly black asphalt in nearly-new condition. (+1 for this route)
Observed that traffic north of Cheyenne was quite light across Montana and Idaho. Could run high beams a lot at night. (+1 for this route)
On this trip the weather we traveled in was relatively good for December. We had 2.5 hrs of 45 mph on slush from Missoula, MT to Kellogg, ID. But the plows were working hard (I had the scanner well programmed) and traffic was nearly non-existent at our early hour of travel.
On return the stretch from Sheridan, WY to Casper, WY had horizontal blowing snow but it wasn't sticking and the light traffic was a plus. Extremely high winds near Cheyenne blew over about 8 empty semi trucks and required 2 hands on the wheel. But... that's Wyoming.
It's possible that the lower overall altitude helped for us to get rain rather than snow during the return. But it was also unusually warm weather in the northwest this time.
This will be the standard route for us in future.