I'm in a good position to answer this. Between me and the rest of my household, our stable includes my Z, a GT-R, a 911, a Mustang, and a Camaro. I have driven all of them for extended periods on the street only, so if you want tracking advice look elsewhere.
I could go into the pros and cons of each vehicle when it comes to involvement, bang for the buck, reliability, street legality, etcetera, but the bottom line is that your car should have one objective: to put a goofy grin on your face every time you drive it. The Z and GT-R, while in different classes, still maintain some of the best driving characteristics in today's street cars. They both make the pony cars feel like oversized boats on the road.
I would have a difficult time placing the 911 among the two Nissans. The most involving and visceral car is the Z, certainly. The most livable and elegantly sporty car, the Porsche. Godzilla, though? Your head will come unscrewed before you reach the limits of that car. Point is this: the 911 isn't necessarily better or worse than the Z or GT-R. Just like you and me, every car has a personality that comes with the good and the bad. If you love driving the Z, keep it. Once you get used to the face-melting acceleration of Godzilla, the drive is somewhat less exciting. Just press the gas and point the car.
I will be boosting my Z.
