R34 Pros: Newer (more reliable i guess), looks better (debatable), has VVT (Neo).
R34 Cons: Fewer spare parts around, insurance possible more expensive, more expensive to purchase.
R33 Pros: Same engine sans VVT, in my eyes just as sexy (series 2 is anyway... S1 lacks that aggressive styling), alot more spare parts than R34, tidy simple function interior...
R33 Cons: Older so more prone to reliability issues, alot of them around if you're after that individuality factor....
The bottom line is price. GT-Ts are probably closer to $30,000 while GTS-ts are sub $20,000.