ZD can be used to Small and Medium consumers, I have see some users purchasing ZD1200 for their homes, where Smart Zone and vSZ is being used for large, Enterprise, Hospitality and Carrier customers. We are using SZ100 for our two Hospitality customers.
ZD is hardware based where SZ is hardware based too, but vSZ is easy to install on virtual environments like VMware, Microsoft Hyper-v, Linux KVM and in AWS cloud, Google Cloud as well.
Regarding features please feel free to download their respective datasheets online.
For a brief techinal understanding please refer this blog,