The best indicator of which 3-ring mechanism you need is how much paper and what size paper you are putting in your binder. This will dictate the size of the ring you need. Whether you want boosters (the tabs on the end of a binder mechanism that help open and close it), will depend on aesthetics and how often the binder is being opened (boosters help keep rings from bending).
The right binder ring makes a difference in the form and function of your custom binder. The size of your ring determines the spine width while the intended use of the binder should determine the style of your ring.