Z-series and X-series KMC chains are two very different quality levels.
Z-series works well, is very low-priced and is reliable, but has very loose tolerances for pitch from one link to another, so much that I have a hard time determining the exact level of wear using any of various measuring tools that references off of the rollers. Perhaps it's just the rollers that aren't concentric, more likely the overall mass-produced construction(?).
At any rate, I hesitate to use the Z-series chain on my more-prized freewheels for harder and higher-mileage use.

X-series on the other hand is race-quality chain with much better tolerances, higher tensile strength and higher pin-retention force. So it costs a few dollars more. Definitely on par with comparably-priced chains and seemingly better value than most.

I will need to see how the new model names compare to the old names. I just bought some KMC 11s chains in the new packaging but their was no doubt that these were of the high-quality grade since I don't think they even make a cheap-QUALITY 11s chain.

7.1 and 7.3 refer to the pin width, and is the outside measurement of the chain. Shimano HG is 7.3mm, SRAM is 7.1mm (these are all 6-7-8s chains).