The Premiums also have a wider driveway than the Preferred by 4-5 feet. Coupled with the fact that, as daverene said, the Premiums have pavement in the back where the tent pad used to be, makes it a good fit for people with long trailers/rvs (the driveway is also flaired out to make it easier to turn/cut as you are backing in). As you pointed out, the location is more toward the middle of the Fort.
Also, there are minor differences in that the Premiums have a square four-sided picnic table for seating with a grill whose cover is hinged to cover it. The Preferreds have the traditional rectangle two-sided picnic table for seating and an open grill. Again, minor points in my book.
By virtue of the fact there are more Premium loops, there is more variety regarding your location within the Fort whereas Preferred are, like you said, all down near Pioneer Hall and Bay Lake.
Oh, and then price, of course, but they're not far apart now like they used to be.
Bama Ed