The 3D is designed for older sites because it is tuned to bring the older nickles, Indian Head pennies and other older targets up into the high tone range when in enhanced mode. Some gold rings and gold coins will hit as high tone as well.
It has 4 tones vs 3 so you can listen to the differences between the foil and tab ranges and you don't have to meter watch as much. Nice feature as you can ignore the mid tones and dig the lo-mid tones if you want to hunt for the thin, diamond rings and not dig a lot of pull tabs ( conversley, you will dig foil). Cool feature for beach hunting to look for earrings and womens rings. Not sure how much you want to pass up at a beach ( I don't ) but you can hunt with it that way. What's nice is that the 4 tones are still very easy to differentiate so there isn't much interpretation.
I purchased one about a month ago but because of all the darn snow, have not had a chance to use it. I have been using my CZ6a since they first came out ( and got another one as a backup) and can't wait to try out the 3D in some older sites.
If you are looking for another CZ, I don't think you can go wrong with the 3D ( or a used CZ 5, 70 or 6a for that matter).
JC