Billings to Iowa Route Question

[pigdoc]

DW and I will be visiting Glacier NP in July, then spending a couple of days in Billings. From there we need to get back to Iowa. I'm looking at some options:
a) Billings to Miles City then thru SD on US12 to I29 to Iowa
b) Billings to Casper then east on US20 across northern Nebraska to Iowa
On the way out we will take the interstate thru SD to Billings to Glacier, so we want a different route back.
Any experienced suggestions would be appreciated. We have a 34' fiver for the trip. Thanks in advance.
Pigdoc

[dakratdave]

you dont specify if you are looking for things to see or just route.may i suggest taking i94 to Medora,ND. Seee Theodore roosevelt nat. park,take in the pitchfork fondue and the musical.