I'm on High Sierra on one Mac and Mojave on another.

Other than APFS, I cannot see any real differences in usability, other than opening quickview and then pressing shift and up to go to the previous file in a list (so you could look over many photographs and select them so as to choose which ones to delete) is now busted in Mojave and will only show the first file selected, making quickview now useless.

In High Sierra and prior (back to Snow Leopard at least?) this would work and show you the preview of the most recently highlighted item, so in a selection of 20 items, if you shift-up select item 21 you see 21. Now you only see item 1 all the time. It's a great annoyance.

So no I cannot think of a reason to upgrade.

I have found High Sierra to be subtly faster on older hardware, and subtly less crash-prone (although neither crash often). So I would suggest staying put absent a reason not too.

High Sierra was a very good release.

I'm on HS too, but finding that occassionaly apps require 10.14+, which is making me consider upgrading.

The new built-in screenshot/recorder feature is nice in Mojave.

Dark Mode :)