I would advise ZFS to use for the backup. Why? Well one simple reason: cheap, easy and convenient snapshots, which are incredibly powerful for a backup server.
Without it, consider your primary NAS has been infected with virusses or you deleted a file/dir without knowing, or for other reason the data on the primary NAS becomes corrupt or absent. Now if you set your backup NAS to backup your primary NAS every night, this would overwrite the existing backup. Hence, if your primary NAS is corrupted, you will corrupt your backup as well.
To cope with this, snapshots are the answer. You can snapshot each day, even every second. It will store each change you make. So initially, snapshots are created instantly (<1 sec) and cost no additional storage space. As soon as you're about to make adjustments, the data on the filesystem will differ from the snapshot; only that difference will cost you a bit of extra storage space. So snapshots are fast, efficient and incredibly powerful if properly utilized.
A common practice for a backup server is to perform a 'cronjob' every night, like:
1. create a snapshot on all filesystems
2. sync all filesystems with the main NAS
3. shutdown the system
The BIOS can be configured to power the system on at 2AM; while at 3AM the backup is done and it shuts down. That also means the power costs and noise/wear/dust is very low.
The primary NAS can also use ZFS, but if you have desktops with storage of their own, then keep the local filesystem (NTFS on Windows, Ext4 on Linux) and use ZFS for backup only. If you want a real NAS, then think about building two of them and let them both run ZFS. Keep at least the backup on a STABLE zfs version; thus no de-duplication and other fancy features that are deemed less stable at this moment.
Another option is to use ZFS as main storage, but use external HDDs on your Windows (a DAS; or direct-attached storage) pc to act as backup.
So the three options i discussed, are:
1) use both ZFS primary and ZFS backup NAS system (best solution)
2) use local storage on your desktop PCs and ZFS as backup solution
3) use ZFS as primary NAS and external HDDs as backup solution
You can vary these flavors, like only having a backup of your most important stuff (docs/work); like one big 2TB external HDD and 6x2TB total storage in RAIDZ2 on primary NAS. All these options are extremely reliable, relative to what protections consumers usually employ, this is gold!
But you may want to think about specific setup and write stuff on paper to calculate the costs etc. Each person's situation is different and requires a specific solution.
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