I grew up and had lived in Flagstaff for a long time. I currently live in Las Vegas. In good weather, the Flagstaff route is preferable. The route is shorter mileage-wise and the scenery is much better. I would recommend cutting off I-15 at Nephi and driving south to Levan and Gunnison on SR 28 and then following US 89 all the way to Flagstaff. An alternative would be to take I-15 to Scipio and then taking US 50 until you can get on I-70 west/US 89 South. The other cutoffs from I-15 to US 89 are more mountainous and winding roads.

You will be on a section of I-70 from Salina to south of Richfield. Take the US89 exit going south. At Kanab you have the option of US89A going through Fredonia and near the North Rim of the Grand Canyon, or US89 that takes you to Page and Lake Powell. Either way is scenic, but I prefer Page and Lake Powell. The road is faster. US89A is a very winding road. US 89 ends in Flagstaff.

In Flagstaff, take the first exit possible to I-40 to avoid driving through town, unless you want to see the town. There is a lot of traffic and are many traffic lights going through town. Exit I-40 onto I-17 to Phoenix. If you want to see some very pretty country, rather than I-17 take State Route 89A through Oak Creek Canyon and Sedona - a beautiful canyon and Red Rock country. Follow the signs and exits for this alternative. This side trip will take a little longer, but is well worth the extra time. In Sedona, take SR 179 that will take you back to I-17. Or, if you are impressed and want to see more of the Red Rock country, take SR89A all the way through Sedona down to SR280. Take SR280 east back to I-17.

Enjoy your trip through some very scenic country.