The main reason for value increases in values of sports cars comprise a few main elements (all of which equate to strong demand, which is the simplest answer)...
motorsport history (and success)
enjoyable or engaging driving experience
popularity - either at the time or later on
nostalgia
provenance
I've probably missed a few, but all things combined usually mean high value. The 260z lacks the provenance (it was the successor not the original), didn't have as much racing success, and wasn't as popular at the time, or now. They provide the same enjoyability and nostalgia, but those things aren't affected by an improved diff or a slightly stiffer shell. If we wanted "better" cars we'd just buy a new common-dore which is faster and more efficient, with more luggage capacity, than both zeds.