Route I-80 or I-90

We will be leaving NW Arkansas heading to Rapid City SD area going north on I-29 and was wondering whether it would be better to go west on I-80 and north on 385 and 79 or go further north and go across on I-90. We won't be in a big hurry as we plan to be gone for 4 weeks and want to see Wyoming and Colorado areas also. I am curious as to the what the 385/79 part of the route is like.

We will be in a 32 ft gas MH pulling a toad.

Thanks,

Bob