They are totally different cars IMO that offer completely different driving experiences. To me the GTR was very fun to drive, but it felt like I was playing a video game more than driving a car where I was giving inputs to the computer without having a direct link to the car. It was unbelievably easy to drive exteremely fast through turns, but at the end of the day it wasn't very engaging. I like the car, love the styling, but I would never buy one as it would get very boring, very fast for me.
With that said, I like the Z06, but I would probably never buy one of those either. If I get a high end sports car it will be one with no compromises and for me that is a GT3.