I picked a city that both of your routes would take you: Beaver, Utah on I-15
If you take I-17 north to Flagstaff and continue north on 89 through Page, AZ, staying on 89 N just past Panguitch, UT you'll come to Hwy 20. Take Hwy 20 west to I-15 to Beaver, UT. This is 355 very scenic miles. We've done it often with a 40' motorhome. No issues.
If you take I-17 north to Flagstaff and west on I-40 to US93 to I-15 north to Beaver, UT, it's 504 miles - longer and not good scenery compared to the above recommended route.
Good traveling to you!
Full-timed for 16 Years
. . . Back in S&B Again
Traveled in a 2004 Newmar Dutch Star 40' Diesel