Better power, ride, interior, stereo, sound deadening, appearance....
Same 5.4L 3 valve engine, same frame, same base suspension and air suspension setups on both the Expy and the Navi. If one had the 4.6 or non air suspension then okay, I could see that.
Interior would fall under style or appearance. That is still the same as any sister car from the same platform. Expy/Navi, Explorer/Mountaineer, Escalade/Tahoe, TownCar/Crown Vic, TransAM/Camaro, Etc. They all have similar interiors but they are styled differently. Look at the interior of a Lincoln LS and a Ford T Bird or Jaguar S type. They are almost identical in each car.
That leaves stereo and sound deadening, bigger wheels and other shit like that.