If you can only get one detector, and are going to hunt the Jersey Shore, get the CZ21. The salt air and water will bust up the CZ3D if used for a prolonged time no matter how much you try to protect it. You can rinse off the CZ21 with no worries. If you are going to land hunt with the CZ21, get one with the 8 inch coil. It can do a very good job on land, and will be slightly less sensitive to smaller lower conductive targets due to it being permanently locked in salt mode ( this mode eliminates the effects of salt water, which is in the same range as some smaller lower conductive targets). It will still hit silver deep and do a fine job coin and relic hunting. Only thing is there is no visual ID and it will be heavier than the CZ3D...but you can hip mount it or mount it on a straight shaft. The stock configuration is not very well balanced...top heavy. But it is built like a tank and can be taken into the toughest environments whether land or sea.
The main thing you need to take into account is how much of your hunting will be at the shore. If you plan to hunt at least 50% of the time there, the CZ21 is the better bet. I will always recommend a water proof unit for salt water beach hunting, especially if it will be your only unit.
One other suggestion is to get the CZ3D, be very careful, and save up for a dig it all used water proof PI unit for the beach. But be forewarned, some NJ beaches are loaded with iron and a PI unit will drive you crazy.
Just remember that beach hunting season is upon us, and when that nice Noreaster hits, you will want a unit that can stand the brutal weather without any worries.