Dan,
If you are referring to the segment for copper/silver and up, I don't know the answer. It has been said many times by Tom D. (and others) that older silver that falls below that range due to being co-located with iron or trash or deep enough that the other CZ's drop it down in ID will come in with a high tone more readily on the 3D. I have to take their word for that simply because I primarily hunt older sites and anything above small foil gets dug. I very seldom use an "ID" machine at the sites I hunt. Being that I live in the country and there are no large towns and consequently no parks or old gathering areas I can get away with my dig-it-all philosophy. Now if I did hunt areas like that, where digging a lot of trash to get what silver is left is not an option, I would certainly consider the 3D because that is exactly what is was designed for.
HH Tom