There are a few differences between Preferred and Premium. Premium has less sand and more asphalt, but there is still a sandy patch at the back of the site for a tent. All sites allow for 2 camping units as long as one of them is a tent. The premium sites also have nicer grills and nicer picnic tables.
Site size varies greatly between all the loops. We have been to the Fort more than a dozen times in the last handful of years and have stayed in Full Hook-up, Preferred and Premium loops. We have stayed in small sites in the Premium loops and have stayed in large sites in the Full Hook-up loops. There are a few sites in the 1400 loop (in the 1430s) that can be tricky to back into with a large TT.
I think everyone else answered your transportation questions.
-Michael