The R34GTT is nice. The NEO engine with VVT-I is quicker too, I think. Interior is much nicer too.
Though, whether it is or isn't worth upgrading from a 33GTST to a 34GTT is still a tough decision.
Personally if I were upgrading from a 33GTST I'd go for something a bit different, maybe the tried and true RZ TT 6Sp Supra or something not so everyday like a DC2 Type-R.
If you do go the GTT (which would still be a great car to own anyway), be careful how you mod it. GTTs can look absolutely horrid - like anorexic 34GTRs - with the wrong mods.