From AZ to Price UT (where it leaves US6) US191 is relatively flat. Sure there are a few grades, such as where it climbs a step in the rock formations, or out of the Colorado River valley north of Moab.
North of Price it climbs to a pass (unnamed), and drops gradually to Duchane.
I haven't driven north of there, but I think there is a climb and descent before the Flaming Gorge Dam, and then another climb to the Wyoming Great Basin level.
An alternative in northern Utah is to stick with US6 to Provo, and take the freeway into Wyoming. But this both misses the scenery around Flaming Gorge, and passes through the SLC urban area.