ZW's have a basic flaw for running more than one train at a time. Both handles draw power from the same core, so if you start a second train, the first will actually slow down. But they are great for accessories as there are four variable handles so you can fine tune the voltage for different accessories with each one. Another advantage: ZW's are really cheap now. In the BAD old days, they were going for a standard of $1 per watt, and I can remember asking prices of more than $300! Around here, they are going for about $75 now.
If it were me, I would either go for the Z4000 or Charlie's suggestion of a separate transformer for each loop, which is what I have done with a new, two loop "pandemic project" layout.
