Ive own boththe Sc300 Manual and the 300ZX N/A and turbo. I would say it depends on what you would prefer.
I liked the 300ZX because of the weight and the aerodynamics of it. The motors in the 300ZX are as dependable as you can find but when something breaks (and trust me someting will as the car grows older) you will find that it is a serious pain in the rump fix. The handling aspects on the 300ZX are nice and the chassis is well built for the curves. I had a full racing suspention setup from Tien and I loved taking it out on the twisties.
I now own a 1993 Lexus SC300 and I live it. The biggest differance between the two is the luxury aspects of the SC300/400. Both of the SC's are tuneable and reliable. When I bought the SC I was looking for a luxury coupe that was dependable and tunable and the SC delivers on both fronts. and the 2JZ motor in the SC300 is the easiest motor to find parts for and installing them due to the size of the engine bay. The SC are nice on the turns but the tend to not like the tight turns as well as the 300ZX. This is due to the weight of the SC platform. Tien suspention parts will help with this and after some gettin used to you'll love taking the turns in a SC.
Like I said its up to you...... Good luck