OpenZFS on OS X

So I am setting up a zvol on my pool to act as Time Machine storage for a number of network clients. I'm using the second method described at https://openzfsonosx.org/wiki/Time_Machine_Backups, with one possible exception: it turns out I can format the zvol as either HFS+ or APFS—either one will work as a networked TM target—and I don't know how to choose. Does anyone have insight into this question?

Thanks in advance,
Dave

My experience with APFS formatted spinning rust? It is slow as molasses.

That's on an external USB3 capable drive -- unfortunately connected to the USB2 port on my MBPro-mid2012 laptop. While it *could* be the USB2 connection, I actually used the exact same disk and connection when it was formatted as HFS+, and got subjectively far superior performance.

I'm staying away from APFS as much as possible now. YMMV...

I had the same choice to make last month, and I went with HFS+. The reason is not particularly smart: it seems to me that until APFS becomes the preferred filesystem to host a new Time Machine backup, there is no downside to using HFS+ as a TM host.