For ZD high availability, you buy two controllers and configure smart redundancy. Each controller comes with 5 AP licenses and when you configure smart redundancy, the licenses pool (each controller licensed for 10). If you have a total of 50 APs then you need to buy 40 more licenses.
With ZDs, one controller will be active and the second one will be standby. In a ZD network, the APs are dependent on the controller and cannot function without the controller. If you have only one ZD and it goes down, wireless will be down until the controller is replaced. If you have a redundant (standby) controller and the active goes down, the standby takes over. There is a very brief outage of a minute or two while the wireless sessions are re-established. Afterwards, the network is 100% functional.
Smartzone is different in that the APs do the work and do not need a controller to function. If you lose the Smartzone, you obviously lose the ability to manage, but as far as users are concerned, everything continues to function without interruption. This does require that you disable auto-reboot of the APs and un-proxy RADIUS if you're using it. Another key difference with the SZ is that all nodes of a cluster are active. So, if you cannot afford to be without the ability to manage, you need to add a second node. If buying additional node(s), you buy the same part numbers as you bought for the first node (another SZ plus another support).