Rick,
The most important thing about selecting a DCC system is to select one that you understand.
The Digitrax systems aren't bad. The Zephyr Extra (DCS51) is a good choice as it has the capabilities to run small to medium layouts, read and write programming features and is expandable through Loconet to easily allow for extra throttles, boosters or other peripherals. One thing to be aware of. If you get a Zephyr Extra make sure it is supplied with the South African power supply. There is something with the US type power supplies that doesn't play well with our local electricity supply. Due to this it may be wise to look at one of the local dealers.
Another system that isn't half bad is the ESU Navigator. It comes with a hand held control that communicates via infra-red. Additional cabs and IR receivers can be added to the system. It's pretty easy to switch between different locos on the fly. The system also has read and write capabilities.
I have no experience with the z21 systems (in either flavor) so I can't comment to much. Sure it runs from smart devices, but I could do that with my Digitrax equipment a long time ago already. That said people who aren't so technically advanced seem to like them and think they are capable and easier to use than Digitrax systems.
My layout currently uses two Digitrax Zephyrs (the older DCS50 model) as well as some UT4's a Dt400 and even some old UT1's.
If you have specific questions about the different models then feel free to ask.
HTH,
Niel
