Not much difference in practice and none that I know of as far as actual "restrictions".
You may sometimes find it harder to level a 39 footer if the site is barely long enough, since sites may rise or dip a bit at the ends even though the central portion is fairly level. And if you like campgrounds with lots of trees and/or twisty roads, you may find the additional wheel base a slight inconvenience.
None of the above applies if you frequent upscale parks with broad roads and wide open sites.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC