If you are going to make a trip of it and stop overnight somewhere, the sooner you jump over to 89 the more interesting things you are going to see. Tons of cool old towns, rundown barns and back road type driving. But you've also got to deal with things like 35mph speed limits in little towns.
Popping over to Bryce Canyon off of 89 doesn't add too much time and is definitely something worth seeing. You can also head over to the North Rim of the Grand Canyon on your way to Phoenix from St. George.
Depending on when you go though, some roads may be closed for the winter so you'll want to double check that.