We have done both, and if it is the castle that you want behind you, wait until you are the castle end of Main Street. However, with the Photopass, why not get one with Main Street, too? Each photographer usually takes 2-3 different shots, so when you get home you can have many different options and a better chance of everyone smiling and looking good.
Also, the photographer who took our castle picture told me that they were no longer allowed to do the Tinkerbell picture with the castle background. She had some excuse about how it wasn't working well and that they were told Tinkerbell might be removed permanently from the choices. I'm thinking it is more about getting people to buy more pictures, one with the castle, and the cute one with Tink in our hands with Main Street behind us. I know that if I had one with Tink and the castle, that's the one I would pick if we were all smiling.
Oh, and they also superimpose Stitch coming out of a hat that we are holding at MGM with the sorcerer hat in the background.
I had to ask for the Tinkerbell shot, but the photographers at MGM were taking the Stitch one for every group.