I think there are a lot of hunters who have used both an Excal and CZ20/21 (me included) to know that the differences in depth/sensitivity to targets is minimal, and it boils down to what uniti is going to give YOU the confidence to be productive.
There are several differences in each unit, which only you can decide which ones would benefit you the most. Below are some of the main ones:
CZ - Dual frequency, Silent search in discrimination mode, manual ground balance, 3 tones, concentric coil design, low tone on iron in discrimate mode when set to 0
Excal - Multi-frequency, Threshold based in discriminate mode, automatic ground balance, multiple tones, DD coil design, null on iron in discriminated mode
So...if you are more comfortable with a threshold based, automatic ground balance unit and like DD coils, then the Excal would be best. If you like a silent search unit (some divers do), manual ground balance where you can set it a tad positive or negative, and like concentric coil design due to it being a bit easier to pin point, then a CZ would fit the bill.
Last thing you want to harp over is the depth differences between these 2 machines. I know that in the areas I hunt, the NJ beaches, the depth difference is a moot point. Plus, conditions dictate which unit, if any, would be deeper and even when one may be deeper, the difference is minimal.