I'd say there are three considerations that require a specific kind of unit. Saltwater and highly mineralized sand there's some that do better than others and some don't perform there at all. In freshwater most work fine but some are more consistently finding the small stuff. When you want to go really deep, but dig everything, that's where the PI shines.
I don't do saltwater. Can't where I am in the US. Or at least it's not worth detecting. Had an Excal but got rid of it to get a fresh water only, pretty much, machine and got the Tesoro Tiger Shark. I also wanted the chance to go deeper than most who hunt the lakes around me, hopefully a little advantage, and got the White's Dual Field. The TS does not like salt and mineralized sand. The PI doesn't care where it is but you dig everything. IMHO the only dual purpose machine is probably the Excal. If you can run it expertly, it's probably as deep as you want to go. It also doesn't care if it's saltwater of freshwater. I have a Fisher, not the CZ, and it's really good in the fresh water. I have no idea how the CZ and the Excal would do on a side-by-side test. If I could only have one machine and needed to hunt both salt and fresh, I'd go with the Excal. Jim