i recon you should keep the r33, just for the fact its built to your liking and everything is done on it (once you put the engine back together). when you say you wont mod the r34 gtr as crazy as your r33. do you honestly beleive it? lol. you'll be pouring money into it, modding never stops once you start. thats why the r33 is good, not much else can be done. also think about the costs of maintaining an immaculate r34gtr, especially when something goes bang! when it comes down to it, if i had an r34 gtr. i wouldnt trust it being parked anywhere for more than 5 minutes. so many stupid jealous people these days.
at the end of it all tho, ask yourself whats more practical, and the big factor of the maintanence of the R33 vs. R34GTR.
(i wish i had an r34 gtr! )