I have been using the cheapest KMC chains for a LONG time, which recently has been the Z series. Then read somewhere that the X series of KMC are supposed to last longer.
Very likely on this forum. The KMC Z series are for single speed transmissions: fixies, hub gearboxes, suchlike; they do not have the flexibility that derailleurs demand. The KMC X series are for derailleur transmission; they are designed for more flexibility. In theory, the Z series, being cheaper to make, should be cheaper at the retailer; in practice unit costs are also influenced by length of production runs, and the last time I bought chains, the KMC X8-xx was still cheaper at Chainreaction Cycles in Belfast (my longtime component pusher because they offer free delivery to the Republic).
As to why the KMC X lasts longer, my guess is that the extra built-in flexibility is worth something, even on a properly set-up Rohloff hub gearbox, when "properly" is defined as "a chainline with a deviation from parallel of no greater than 1mm".
I've used the top of both the Z and the X range as a chain running parallel to within manual measurement deviation (in theory nil, in practice no more than 0.5mm), and the Z works fine and in my blueprint installation the difference from the X is not great; I have much greater experience of the X, which I fit and have spares of, and make gifts of to pedalpals still in the neanderthal age before hub gearboxes, who never knew there were chains that good and long-lasting until I gave them one of mine. That makes the key consideration price on the day you buy the chain, and CRC has periodic sales in which the X routinely leaves the Z for dead on price, in which I stock up.
On your installation, George, where I seem to remember you have a 5mm-off-parallel chainline, it appears to me at least possible that your mileage gain from the KMC Z to the KMC X (for newbies: given that within those ranges you swap quality for like quality, the higher numbers being the higher qualities, and the -xx indicates plating protection, the higher the number the better as in KMC X8-99*) could be much greater than the gain I found with a practically perfect installation operated only in favorable conditions.
*More, where in earlier years on this forum, you will find references to a KMC Z8, that's an error. At the time I first made the comparison, the Z7 was the top of the Z range while the X8 had an extra feature and was thus the top of the X8 range, and the Z7 was often called the Z8 by analogy but incorrectly. In any event, we compared them as like to like because they were what was available at the top of those ranges. You can't go wrong buying the KMC X8 whereas if your chainline is not perfect the Z7 (or Z8 if today there is one) may in fact be inferior to the X8. I personally find the KMC X8 the perfect balance between price and longevity on any Rohloff installation, and of course essential on most Rohloff conversions on derailleur frames where the necessary jockey wheel is an invention of the devil.
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I've said this before, so I'll just mention it for the OP. The chain is the cheapest part of the transmission and should be replaced before it endangers more expensive components in the drivetrain. I replace mine at about 0.5 wear on the gauge, eyeballed because clearly the common 0.75 gauge won't fully engage with the chain at any lesser wear. Alternatively, if you're a high-miler who intends wearing the chain, sprocket and chainring at the same rate because that is one easily understood path to lowest replacement cost per year, alternate say three chains. Martin and others have several times provided instructions you can search for. Reading passim in these threads, you will have discovered that many of us find the Hebie Chainglider a useful substitute for chain maintenance without cutting into impressive chain mileages!
