Should I take Hwy 70 to 89-- or Highway 20 to 89 In Utah??

I've got an older rig. 1992 Monaco Dynasty 31' 5.9 cummins 230hp. No Pacbrake and will be towing a UTV trailer with Rhino onboard. I've traveled the road into and out of Laughlin Nevada in winter time, yes, it's a slow climb but no problems.
However, I'm concerned with Highway 20 and the steep downhill grades. Would it be preferred to take 15 to 70 and then hit 89 ??

Thank you in advance for any assistance.

Mike