Weather you use z1 or z2 (which has the same functional requirements, benefits and risks as Raid5 or Raid6) depends mostly on what risk you are willing to take. Performance wise the difference is mostly that z2 can provide slightly better read speed while being slightly more taxing on your CPU. Both setups are geared towards redundancy in favor of high performance IO or speed. In both cases the biggest risk driver for a complete array failure is the time delay before a failed drive can be replaced and the size of your individual disks.
With z1 you can lose one drive. If you have 3-4 drives of average size and can replace the failed drive quickly that's probably an ok level of risk. However if you have a week of delay before your failed drive can be replaced or if all your disks are big, as in 8 TB+ range, it might be worth looking at z2 The reason for this is because the time spent to rebuild your array increases with disk size. Hence even if you replace a single big disk quickly the chance that a second drive may fail while an array is rebuilt increases as the process takes longer to finish. Similarly if you stick with z1 but scale up to say 8 drives, even if the drives themselves are small, then you also increase the risk of a critical failure because there are twice as many points of failure present while you are waiting for your replacement drive and/or rebuilding the array.
It's worth noting that you can run both of these setups with a spare drive present that automatically takes over for a failed drive. However this only removes the delivery time component from the risk equation. Also there are horror stories of people who have had spare drives present on a numerous z1 array, as in many disks, only to have the spare and another drive fail as the raid is being rebuilt. You should set up SMART tests (both short and long ones) on the spare as well as the raid disks and check the SMART data to mitigate this risk, but even then you can be unlucky. Rebuilding the array taxes disks more than your long smart test does and can trigger a drive failure that SMART was unable to predict. Hence why you often wan't to go to z2 first and then add a spare if you still find the risk to be unacceptable or have a configuration that's in-between the next natural array setup with respect to the number of disks used.
One minor disclaimer here. While I have used Freenas/Truenas for a couple of years now I am not an expert on the system. That said I have used Raid for several years prior both on my own equipment and at work and the general principle behind array sizes and redundancy has remained pretty much the same for a long time. The only real difference being that solutions like Truenas and cheaper and better disks for regular consumers have made it much more viable for us :)