My preference for a robe or house coat is for a coil zipper.
But for a choir robe, I would recommend a molded tooth zipper that is a 2-way separating. A 2-way zipper allows you to sit comfortably especial at an organ.
They are easy shortened from the top by adding new top stops. I have a video above to help you.
Click on the picture to select your length & color: