You're probably way past this by now but I wanted to throw my two cents in. I actaully had both the molds done and the 3D imaging when I started. The main advantage to the 3D imaging is that they don't have to wait for it to go through the mail, everything is on the computer with the 3D imaging and makes the process quicker. I had to have a second scan done after 10 trays and that time they just did the 3D imaging and not the molds.
I think you should wait until the first round is done and plan on making the full payments. Most likely there are refinements included with the price of the Full and there aren't with the Express. I would personally wait and see if you want and/or need refinements.