I did not notice a whole lot of differences... but I only looked at the 15th for a few minutes before I opted to go with the 16th. Since the 15th is 4 years old I imagine a lot of the pharm protocols have been updated. I also recommend shelling out a few extra bucks and going for the two-volume set... much easier to carry around the one volume you need at any given time. Since it is organized by systems you can usually get away with only one of the two volumes.