RAIDz1 is just the ZFS version of RAID5. Since TrueNAS uses ZFS, RAIDz1 is what you’ll be using (or RAIDz2 if you want an additional disk’s worth of redundancy at the cost of a disk’s worth of storage).

RAID5 isn’t applicable to your situation. You’d have to be using a different OS, with a different default file system, for that question to matter.

What’s the difference between RAIDz1 and RAID5? It’s complicated. Without getting into the underlying specifics of how ZFS works, in short they’re two different ways of achieving the same goal. To the end user, the differences are immaterial.

As an aside, if you’re going to be using TrueNAS, which relies on ZFS for its storage technology, you’re going to want to check if your disks use SMR or CMR. See this post for details; www.truenas.com/community/…/WD-SMR-iX-Statement/