haha if the Z was designed as the sports car not the GTR what happened there ?? i dont know facts but my impression is that the GTR WAS designed as the sports car, just because the NA models came out first doesnt mean the GTR wasnt in developement. plus in my eyes the 300zx was there attempt to compete with european supercars, look at the styling, and again i dont know facts but GTR's seem to be in a lot more racing situations that 300zx's dunno how it was back in early 90's though ?
basically i see the GTR as what was built as the real race car, for japan, not nescesarily to go head to head with 'supercars' (i dont see it as a supercar, its just better than a lot of them ) and the 300zx as there supercar attempt.
as i tried to say, this is just how i have percieved the cars, not based on fact or even opinion really. perception
but i wouldnt call a GTR a supercar, even if its better than a porsche or whatever from the same era.