I own an R34 GT-T with an R33 GTR motor in it, but i sell these cars for a living. The R34 is a far more apealing car in terms of looks, styling etc. but performance wise and durability, the 2JZ is far better than the RB25. If you were comparing to a R33 GTR i would probably go for the GTR just because 1 its a GTR and 2 i am a nissan man, never owned anything but a nissan. Also the driving/seating position in the supra is pretty crap, you sit down too low, cant see over the bonnet properly (btw i am 6ft1) and you have heaps of blind spots due to the design of the hatch.
Take the supra, better quality car all round but make sure you go for the 96 model with the bigger brakes, better interior, recaro seats, 17" wheels yada yada yada we have one for sale at the moment for just over $30k immaculate 6spd with sexy work wheels. check it out if you're interested (kamikaze motors, nunawading, vic)