Slabs
Any road is good, some better. Just so long as you stay off the slab. There is nothing more boring than a steady 75 or so on blistering hot concrete with carloads of of kids and big trucks passing you 3 feet away. Then, when you need a break, the exit ramp brings fast food, snotty kids and irate parents. You can always use the rest areas along the slab. Usually, the toilets are cleaner than the urinals but don't hold your breath, well, you may have to, to avoid becoming sick.
Back roads equate to good mom and pop resturants, little gasoline stations and plenty of things to look at or take pictures of, basically a never ending change of scenery. Slabs are great for quick trips to anywhere but backroads are always better.