Is when hunting potential older sites for older coins, and only digging high tones. The 3D designed to bring some older silver coils as well as smaller gold coins into the high tone category, based on it's ability to analyze the older coins make up, is a plus. A minor down side is that if a 3D is used in enhanced mode in trashier, modern sites, some trash will hit as high tone as opposed to mid tone when running in normal mode.
If you are prepared to dig all repeatable mid and high tones when hunting with the CZ5, then there is no difference. If you want to cherry pick high tones only and want the ability for some older coins to hit in that tone range, then the 3D is the unit for you. In addition, you can cherry pick foil ranged targets by tone (some smaller engagement rings fall into this category) due to the 4th tone on the 3D. You can do this on the CZ5, but need to watch the meter since foil ranged targets hit as mid tone similar to tabs, and do not have a separate tone.
It really depends on what type of hunting you will be doing and where. If you are going to be hunting more trashier spots for coins and jewelry, then a 5 inch coil should be in the arsenal. Hunting salt water beaches and wide open fields/pastures, then the 10.5 inch coil should be in the arsenal.
So with the money you save on getting the used CZ5, you could probably get both coils.
As far as depth comparisons, all the CZ5's are made in Los Banos and have the components and quality assurance procedures implemented there...which apparently is an advantage. A newer CZ3D may not have this advantage and there are reports that some newer 3D's do not have the same depth capabilities as the older built ones ( and the older model CZ's).
So the first determination is, where was the 'new' CZ3D built, and how does it air test. Seond is what is the price difference between the two, and can that difference be used to buy another coil, or a pin pointer or some other accessories. Third is, what type of hunting will you be doing, and in your mind, does the enhanced mode of the CZ3D trump the CZ5, and will it be used to your advantage.
I had a CZ3D, 1021 serial, tuned by Tom D. Was a great unit, but for me, it did not give me any advantage over my CZ6a's or CZ7a, (depth or ID wise), since I dig all repeatable mid and high tones anyway.
Just my 2 cents.
