Since there is much more to see on the northern route, and since you seem to have more time on the way back, I suggest you you come to Los Angeles on the southern route through St. Louis to see the Arch and work your way down to the 40 and see the "big hole" as Dave suggested. Take the Vegas exit off the 40 in Kingman and drive accross Hoover dam. If you have time stop and take the tour, if they still have it after 9/11. There will be some long boring stretches in there, but most western states have that problem.
On your way back, take the northern route. Why drive back the same way you came.