Jama at Cedar Breaks
I have done both. Multiple times.
Fact is, you have to climb - regardless - in southern Utah.
You will have a big, but scenic climb out of Kanab on US 89.
Via Zion -
Zion NP is spectacular, but insanely busy.
The problem isn't I-15, but Hwy 9.
It has heavy traffic and almost no shoulders.
Plus a tunnel in the park that you have to beg a ride thru.
I have been in touch with park officials for years about this.
They have shown no concern even though this is especially dangerous for women.
Sometimes you get a ride quickly, sometimes it takes quite a while.
Via Hwy 14 -
Both US 89 and Hwy 14 have limited shoulder - US 89 is better.
Traffic is moderate - more as you approach Cedar City.
But by then you are screaming downhill.
Cedar Breaks (more climbing) is a beautiful option to Zion.
But you are not going to Bryce Canyon NP?
Which means staying on US 89 to Hwy 12?