I would go with the GT-R (at least for now)....
Although this is all speculation, I think the extensive testing that nissan has been doing and the new technologies that will be implemented in the car are going to make it an awesome performer overall, track and street. Also, there's the advantage of AWD in adverse weather, and I believe that it is supposed to be a 4 seater as well. Though the cobra will undoubtedly be the acceleration bang for buck, i highly doubt it'll be as fun to track as the GT-R. Plus, if a highly tuned R35 GT-R sounds anywhere near as good as an R34.......drooooooool......