Tom,
For the stacked there are three different types. One requires that you run one piece face up and the mating cut face down, the second runs both face up and the last (like the Freud offerings) runs both face down. For the first two you have more possibility for surface mismatch on the front of the doors. Also, with the first type the groove part is used for both cuts so it will wear faster.
Comparing to the matched sets, the stacked type can not be shimmed so once they are sharpened the fit of the joint will not be as tight.
For the stacked there are three different types. One requires that you run one piece face up and the mating cut face down, the second runs both face up and the last (like the Freud offerings) runs both face down. For the first two you have more possibility for surface mismatch on the front of the doors. Also, with the first type the groove part is used for both cuts so it will wear faster.
Comparing to the matched sets, the stacked type can not be shimmed so once they are sharpened the fit of the joint will not be as tight.