The GTD other than the motor and some v subtle suspension tuning differences (believe mainly on the front to accomodate the heavier diesel motor) is pure GTI. The press says its a little softer than the GTI but i put this down to it probably being a little more nose heavy and so more prone to understeer. Its bloody firm enough so if the GTI is any firmer I am not sure id like it v much.....GTD is acceptable but I wouldnt want it any firmer.
The 184 TDI I believe has bigger injectors...not sure on the turbo this time around. I did read somewhere it has variable valve timing too....bit odd for a diesel but might also account for its slightly more free revving nature vs the regular 2.0 TDI.