Well.......
The SC is a more comfortable ride, larger car, and more power.
The Turbocoupe has less power, less weight and better on fuel. The turbocoupe is easily modded with intercoolers and bigger turbo or just raising boost. Bad points are it's a turbo, oil is critical, turbo timer is a biggie (lets turbo cool down), and car has solid rear axle.
The cars have a lot in common since the Turbocoupe has ride control and a large following. Parts are available since they made A LOT of these over the years.
Get an 5-speed if you get one, there a better engine and bigger turbo (auto's couldn't handle the power).
The Turbocoupes can reach over 400 RWHP fairly easily with just turbo mods and better cylinder head. I have a turbo Mustang and it is FAST!!!!! (YES, 4 cylinder turbo).
So, decide what kind of car you want. If gas mileage is important (to many it is) the turbocoupe gets about 30mpg on highway!!!!
Good luck with your decision.....make sure the other car is worth it!!!
Don