Traditional veneers require slightly more tooth reduction due to numerous reasons.  Some reasons can be due to tetracycline staining (taking tetracycline as an infant), excessive fluoride intake (such as those people located in Grimes Iowa--remember some fluoride is great for the strength of your teeth and bones, but excessive amounts causes discoloration and defects in the structure of the teeth and bones)  These veneers can also be great for excessively rotated teeth where braces are not an option.   Due to more reduction of the teeth, traditional veneers can be  more realistic as they may be more translucent on the edges and are able to mimic real ideal teeth in a more natural manner.