Take into account that Zion has a tunnel and a ton of switchbacks as you exit the tunnel. If you're too high, you pay more at the entrance station and they close off the tunnel so you ride down the center line. If you are towing and longer than about 55 feet, you will need to unhook so you will need a second driver.
If those are OK, then the Zion route is definitely prettier!
Otherwise you take 89A to Fredonia to 389 which turns into 59 and leads back into 9 and finally I-15.
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles