I'd go the GT-T purely on looks. Looks wayyyyyy better than a Supra, also, R34s are rare, compared to Supras.
Performance wise Supra would win.....but that's what mods are for. I'd go the Skyline anyday.
Oh yeah, this Nissan, Toyota stuff......LoL both are Japanese Manufacturers, kinda like comparing Holden and Ford. You can't say for example, "the skyline is better because its a Nissan" or vice versa. Same budget production methods.