Since Scott’s Bluff is just north of 80, I’d take 15 to SLC and turn right. After that you just have the grades east of SLC which are significant but not difficult.
40 to 25 is a good route but I’d watch the weather because Ive driven in some wind events on 25 significant enough to turn over RVs and tractor trailers.
Even though 15 to 70 is the most direct geographically, I wouldn’t bother with 70 because of the grades unless I wanted to see something along that route.
2016 Ventana 3427