which model? R34?
Ok here goes.....
R33 and R34 are pretty much the same. Being general here! Not going into specifics.
GTR has the RB26DETT twin turbo engine and active 4 wheel drive. 276bhp standard.
GTS has the RB25DET single turbo 2.5 litre engine with 250bhp standard (more like 230bhp though) and rear wheel drive unless it's a GTS4 which has permanent four wheel drive. Also there is a RB25DE that has no turbo.
GTR's are much nicer inside, have better suspension, different gearboxes, better brakes, ABS etc. ABS is optional on GTS's. Also the bodywork is different on all models. R32 onwards, the GTR's have flared front and rear wheel arches, bigger bumpers, different spoilers etc.
Depends what you want really. If you only want around 350-400bhp then you may as well just get a GTS and spend a bit of money making it look better. GTR's are easier to get big power, but if you're like me a GTS is a better option. Cheaper to buy, run etc.
James