Actually the North Rim isn't that hot (64 deg average in June), it is at much higher altitude than the South Rim and generally the snow isn't even cleared until late May. Not to take away from Colorado where we spent last summer and several others besides that over the years. But for spectacular sights you cannot beat the Grand Circle/Loop. Some of the parks in that, are hot: Zion/Arches but others are not Bryce for example.
And to really appreciate Colorado you need to do some of the 4WD backcountry roads like the Alpine Loop. Rocky Mountains NP is a lesser Glacier too.