The simplest way to explain it, is simply put as thus:
The Fan Preview Night is set as a form of dress rehearsal if you will, that is for the sake of being the first official guests to be at the event (unlike a TM Preview event, which is happening).
While the Express option allows you a chance to attend a special panel with the main two most associated with Hollywood's A&D, John and Chris, the overall fan preview night gives you the chance to relax and have fun.
I think Orlando might be doing something similar, if I'm not mistaken (but
@Legacy might give a better answer in this regard) this year, but later on in the course of the event.