I have the older CZ-7, it uses 3 tones, just as the CZ-20 does. Both work well in rain, but only one is truly splash proof. I use the CZ-20 often in rough terrain on land, it breaks down easier and is hipmountable. Sometimes this is a great advantage whether on water or land.
The CZ-20 is better in mineralized ground than the 1280 but since I don't work salt water areas you'll have to ask those who do. From what I've read of the 1280/CZ-20 users, the CZ is the better machine.
The tones are good indicators, but serve best only when you are cherry picking a site. In rural, older sites where you think there may be good targets, I am not sure one more tone would be a factor but I've not gone over to one of the four tone machines yet.
I also use a 1235X and will probably upgrade that unit before getting another high end machine. Possibly a 1236X2. My advice is get something rugged and sensitive in all environments, but unless you are actually going under water, it does not need to be a CZ-20. Hipmounting is also a big plus on long hunts.