I'm no expert, but from what I've read...
Both are great. XF (also referred to as V6F) is a little more accurate and listed as a super rep. ZF is still great and NWBIG I think, but apparently slightly less accurate visually/externally.
The other difference between the XF and ZF is that the XF uses Miyota 9015 movement whereas the ZF uses A2824 movement. People seem to like the Miyota movement with the exception that it can sometimes be loud! I don't have one yet to measure how loud, but it's a common complaint. Apparently a drop or two of oil can make it much quieter.
So it depends what do you prefer? A little bit more visual accuracy with a potentially louder movement, or slightly less accurate but quieter movement.
Another option is the XF/V6F version 2 which uses A2824 also. I noticed that Trustytime still has it listed and can apparently source it, maybe other TDs can too. The V2 is still listed as a super rep but apparently has slightly weaker lume compared to the newer version.
I could be wrong but from memory I think I read that nobody makes LHD Pelagos's anymore? Although it may be possible to pick one up on sales section with a bit of luck!
Edit: Also when it comes to this watch the main QC points to watch out for are the alignment of the markers, and datewheel.