How do you live without a phone? :h
It's not really going to matter which phone you have, though I think the Verizon may have a better signal/coverage in Montana than the disposable....so that may be a good reason for her to take the regular phone.
I think you can also get yourself a phone, for $10/month, added to your plan with the other phone (unless it's a non-contract phone)...though you would have a contract for 2 years ($240)...but, you may find that it's useful more than just this trip.