In case if the appearance is in the first place, opt for a four-prong ring. This is especially true for non-round and smaller stones (less than 1 carat), as the four-prong setting will make them look a bit bigger due to a higher level of brilliance and less metal coverage.
A six-prong setting is a better choice for those concerned about security; however, you should keep in mind that it also makes your stone appear a bit smaller. This type of setting looks perfect with round stones as the construction of prongs that are evenly distributed around the gem create a full round effect, while a four-prong setting gives an angular effect making round stones look more square.