its a pretty tough choice even as only car.
miata does get noticeably better fuel economy and unless im mistaken, lower cost to insure.
ND as a track toy (if thats your thing), I imagine lower weight and more efficient fuel economy lead to lower cost of consumables. but the con is its really hard to store stuff like another set of wheels and tires. also the initial cost of a ND club trim with brembos/recaros approx $5k more in price. Thats not even factoring the cost of an aftermarket hardtop..
but yeah this is the one of the hardest choices to make. If I ever replace my fiesta, with a newer car, itd be between the 86 and ND. slighly leaning on the ND because it wouldnt be my only car and already pay higher insurance.