Skyline is heavier, yes. But, thanks to AWD, it'll be better when exiting corners, since you can step on throttle sooner than on RWD layout.
If there would be a battle of full out tuned cars, in the terms of performance, R34 is better match to 4th gen Supra than R33, since R33 can not reach 900 bhp where as R34 Nür edition reaches 904-913, depending of turbocharger.. on the other hand, Supra is lighter.. and with 900 bhp it'll easily lose traction in corners and thus it's forte would be the straights.
No matter what point you look at it, I believe that GT-R's and Supras will be equal. Skylines would win on the twisties, whereas supras would be faster in the straight thanks to their better aerodynamics.
Thus, the chances are 50/50. both have potential to be the winner, but in the end, it's all about driver, not the car. Driver is the thing that makes the car fast. It's up to driver if his/her skills are good enough to utilize every good side of the particular car.
Thanks for listening my theoretical ranting this long.
and before anyone asks, I am member of Skyline Squadron. And no, I'm not participating this race, since I'm not one of the aces of our squadron. That honour belongs to those who are faster than me in my feeble 700bhp R32 GT-R.