I think it is largely a matter of preference. Most people will tell you they are both good transformers.
While I understand what you are looking at with your math, the transformers really do not work that way. There is only one physical transformer inside either one. So whatever channel needs the power gets it.
Most folks do not use a ZW for more than two trains. The "B" and "C" controls lack a direction/whistle control. I use mine for switch machine power and uncoupling track power.
I prefer the ZW because I do not care for the controls on the KW.
Specifically, on the KW you have one control for the whistle, that is shared by both "channels". So you cannot blow the whistle on both at once. I also find that the pot metal control shaft is failure prone.
Whatever you decide to get, I suggest adding external circuit protection. Circuit breakers for postwar trains, and TVS if you are going to run anything with electronics, including electronic horns / whistles.
My suggestion: Try out the controls, and picture yourself using them. Get the one(s) you like better,
