Not too experienced, but I think If ur not into drifting, want power n handling, test drive both machines, and Im sure you will be happy with the GTT. U want unreg. & white, so thats another plus for the GTT, coz u can join the white R34's, Satigue & Driftmeister. When u see a couple of White R34's on the steet, its like watching angels amongst humans, beautiful.
But, if I were u, try n test drive both machines which u know are in good condition, then before u buy one you can tell the difference. Try to get someone who is well experienced to view the car with you, so u wont get a japanese reject rip-off.
Also, I think u sould do the bare minumum modifications to it, coz u dont want to turn your GTT into a Mutant-steroid muscle car. Mine is done up averagly, and I spend RM50-80/day on petrol for daily driving. GTTs are not monsters, they are more practical works of are with lots of grunt already.
Thats of course if u choose one over the sylvia in the 1st place...