I think the CZ's ( 5, 6a, 70 and 3D) are some of the best all around detectors ever. Once one knows how to use them, run them wide open discrim and listen for to the iron, you can pull out lots of good targets among the iron. When you start to up the discrim and not hear the iron, that is where you run into trouble.
Saying that, as primarily a beach hunter, I use my 6a's and not my 3D since the 3D has a speaker, and the 6a control box is weather resistant. I hunted with the 6a over this weekend, and that thing got splashed and sand all over it. A simple rinse on the face plate under running water, a wipe down, and all is well. I would not be inclined to do that with the 3D. Also, beach hunting with Cz's lends itself to dig ALL mid and high tones, along with some low tones that are deep. So, the 4th tone of the 70 and 3D is moot for beach hunting in my book.
At some point I am going to take the 3D to the beach, but when I do, I will put duct tape over the speaker grill.
If I were you, and are looking for a CZ to beach hunt with, I would look for a 6a or 70 before the 3D since they are much more weather resistant(remember...they are NOT water proof). A used 6a or 70 would cost less too.
On the flip side, the 3D is more tuned for the older sites and could be a difference maker in old sites where it would bring older nickles, Indian head pennies and some gold into the high tone area. You would possibly have to dig many more mid tone targets with the other CZ's to get these types of coins.
I am heading out Friday to the beach for only a few hours, so I may take the 3D on it's maiden voyage there. If so, I will post the results. Will use the stock 8 inch coil too as I want to see how it works with that prior to putting on the 10.5 or FZ12 coils.
JC