Welcome Seijirou,
Are you Japanese?
Any way, there are many deciding factors on the engine, cost, weight, power, etc. but in the end it's whatever your personal preference is. I chose the american v8 because of parts availability, pep boys, mail order, etc.
Also, power is easy to make vs. making 500hp in a supra engine, you need new turbos, more fuel, more computers to control it, etc. etc. and after all that, the engine is more like a race engine, rather than being streetable.
Check out some of the LS1 engine owners here, making much power with just an intake and cam swap!
I'm sure an SR20DET would fit in the Z32, but the car is heavy isn't it?
Owen