I have owned both and i would have to say it depends on what you want the car for. The 34 is a great car but needs a lot of money spent to get big power reliably, the supra on the other hand has a totally bullet proof driveline and its not just the 2jz engine, its the 6 speed getrag which comes standard not to mention the 4/2 pot brakes and huge rotors on the later model supras. I believe the supra to be a better handing car than the gtt as well, it only on old supra's with perrished bushes that this will not be the case(who cares as most will run coilover's at some stage). The supra will put the power down much better as well and can fit 295's under the rear standard and 305's with just a rolled lip on the guard. My cousin is currently putting down 327rwkw in his supra on stock us spec turbo's with just an exhaust, filter, cooler, 044, 680cc injectors, avcr and a power fc. This a great figure in anyones language, especially on stock turbo's. Those figures are just scratching the surface as there are now many 800-900 rwhp supra's on stock internals in the US and our fair share growing in Australia as well.
All in all, the gtt is a great looking car though so is the supra. It's just that the supra is a much better built car in every area except maybe interior where they are both full of cheap looking plastic.
Supra all the way.