Internally threaded belly button rings are superior in their design. The threading is tucked inside the bar and not visible from the outside, this means the threading can not rub against the tender piercing and are less likely to build up bacteria around the threading. For this reason, most new piercings are performed with internally threaded belly button rings. They are easy to wear and easy to keep clean from bacteria.
While internally threaded belly rings are often not as cheap as externally threaded piercings, they are more comfortable and less likely to build up bacteria around the threading preventing infection.
When body piercers perform new belly piercings they prefer to use internally threaded jewellery for maximum hygiene purposes.