Our most direct route from Colorado is I40. The truck traffic from Albuquerque to Flagstaff is very heavy. The road is also pretty rough. As previously stated the weather around Flag has been really bad as in 2' of snow. The traffic between Flag and Phoenix is heavy with many aggressive drivers. We have started taking I25 south to Hatch, NM and I10 to Tucson. I25 in southern NM is a really easy and nice drive.
The difference between the two routes is you maintain the altitude between Albuquerque and Flagstaff loosing it south of Flagstaff. On I25 you gradually loose altitude entering eastern Arizona.