Well considering that it depends on the race, an all out race, the skyline is going to win and I'll tell you why. Being a fully modded car means also taking stuff off, or out of the car. When doing your research, you'll notice that the fastest supra (and I mean all out not a 1/4 mile) in the world has a (skyline)RB26DETT motor in it. and the fastest Skyline in the world is not AWD for the very reason of weight. It has a (Supra) 2JZ-GTE motor in it. And the Fastest RX-7 in the world also has a supra motor in it as well. being a completely modded to the teeth and every possible thing you can do, it's going to be a skyline.
When people do engine swaps like then it may be for a number of reasons. Maybe they like, the RB26DETT engine, but like the Supra looks better. Or maybe they want attention like Top Secret. Who knows. I like the power of Supra engines, cause once those cars go, they are GONE. And from what I know the 2JZ-GTE engines are easier modded. But suspension, and computers wise, the Skyline is more advanced. Put it like this. The Skyline is more of a track car, and Supra is more of a drag or top speed car.

there is no real comparison between the supra and skyline because they were made to do different thing.