[kyriian]

One of the things I'd really look closely is the local autox venues. Bigger venue the Z06 will be more advantageous, power is power
Smaller venue will benefit the MX-5, nature of the smaller lighter car.

Between them they are so different, yet so similar, so i'd point out what was issues to me in both may help your decision?

The Z06 is a tricky beast, but the combination of the good diff, big tires, longer wheelbase, makes it less terrifying than it sounds. Driven two of them before I settled on my Z4M. It does lack modern refinement. It may not be a problem for you but I find that early 2000s American interior disgusting and cannot stand being in it despite the fun and enjoyable driving dynamics. Of course, it will be a future classic and if you are already planning resale, you will more than likely make money with the car.

The ND is tiny, it felt smaller than both my old NB and NC on the inside and I am not big. I do fit fine, but I know friends who have had issues fitting in it comfortably and it maybe a problem for you as well, so I'd really try to sit in one to see if both you and your son will fit properly. But there is no better car new today to appreciate proper car control and weight transfer, like RM7 said, it does not require crazy skills to drive fast, its not so much that it is easy, but far less terrifying to push 100% without as much fear of it biting back, while offering all the modern trappings that the C5 Z06 will never have. Let's not also forget, you can open the top and just enjoy a million miles of blue skies.