Just remember taht many manufacturers starts from a standard size and then make alterations to fit you, so if you are close to a standard size you might not feel the difference between the standard size and the custom made jacket. To get the canvas jackets as stiff as possible, the manufacturer usually use one or two layers of canvas or one or two layers of superlining in combination with the canvas. Only the experienced shooter can draw the full benefits from an extremely stiff jacket as the jacket will be so stiff that it will alter the position of a shooter unless he is used to his position. Therefore beginners should go for the cheaper double canvas jackets and later upgrade to the stiffer third layer jackets when he is experienced and comfortable with all the positions. There are two major materials that are used for jackets and pants - leather and canvas.
Leather is just normal leather that is usually lined with canvas.
Canvas is a coarse cotton fabric that is made in a way to make it as stiff as possible. The leather jackets is better suited for outdoor shooting as it doesn't get as affected by moist, while the canvas jacket is much stiffer and give better support and stays stiff for a longer time. There is a hype among new shooter around the world to get new equipment when they start out. I believe that is a waste of money. A beginner doesn't know what is good for them and doesn't have the shooting skills.