You're right, Everyone's definition of value is going to be slightly different. I could say to someone that I think a Mercury Tracer and a Lamborghini Murcielago are equally great cars as well. However, I believe there are some underlying priniciples sports car buyers use to judge their purchase. Some of which I believe are looks, hanlding, straightline speed, depreciation, exclusivity, running costs etc.
While looks might be argued by some, and straightline speed is obviously in the Z06s favor, the NSX adds value so much more in every other category. If were just looking at it from a financial standpoint, it will be even easier to define which car provides more value, just based on the costs and depreciation expenses.
(FYI, I'm arguing just because it's fun..)
"Sport car buyers" do not all have the same list of attributes, and provide like weight to those attributes as you and I might. A Torque-loving quarter-mile Ford Cobra buyer is no less of a "sports car buyer" than a Mazda Miata auto-cross junkie, even though those two cars have vastly different characteristics in pretty much every category you listed. Likewise, a Z06 buyer is no less of a sports car buyer than a NSX buyer just because they have a different list of attributes and weight to those attributes.
From a financial perspective, I would have bought the McLaren F1, Ferarri F40, F50 (and lots of other Ferarris..) driven them once or twice, left them in my garage and they would have APPRECIATED significantly.
All I'm saying is, buy the car and enjoy it! I've had and driven lot of cars..... and probably have lots more. No car is 'perfect' (except maybe the McLaren, Enzo, F430, GT3... oh nevermind...)