Are you traveling for pleasure? Or work per say?
Sometimes taking the steeper slower route is nicer than the easy flatter route if you will. You also do not say how steel grades are! Some a 5-6% grade is steep, others like me, 15% gets my attention, 20+% is steep! Interstate freeways max out at 6% generally speaking.
Issue with this, is you need to know how steep a grade your rig will do before stalling out. If you don't know this, along with how steep the grade is, then fla5er might be better.
I will admit I have not answered your question, hopefully some food for thought!
Marty