After driving one and have a good nose round, only you can't make that decision.
I think a 350z is possibly a better prospect of you wish to modify, ie wheels, suspension, interior engine, exhaust, subtle body additions as there's way more options due to aftermarket support. To keep and leave fairly stock a Z34 Nismo would be a better choice. I find with a Nismo the more you do to them, makes them almost stop being a Nismo as they certainly won't look like one if you swap out even just wheels and mess with the body. If you're not concerned with keeping a Nismo looking like a factory Nismo, you might as well saved yourself the 8k over a GT and spend it on wheels, exhaust, suspension, tuning and seats resulting in a quicker and better handling car.