There are quite a few differences between the two specs Paul, obviously things like interior trim are very different, colour of headlining, shape of steering wheel, seat bolsters and other odds and ends of trim.
Outside there are the very obvious body differences in the bigger bumpers, half arsed side skirts (and LED tails if yours is a newer GTD) amongst other differences.
The GTD has the XDS electonic diff like the GTI apart from a few very early ones, the suspension spec is unique to the GTD and I'm not quite sure whether the GT has the same size (312mm) brakes either(?).
A friend of mine has a GT TDI company car in black with DSG (currently off the road following a crash by another driver in the snow) and it's a fine car, plenty quick enough for general road use, very comfy and just like the GTD/GTI/R or whatever, it still has that unique Golf feel about it.
At present you can get a very good deal on a GT TDI unlike the GTIs and GTDs so they look even better value for money.
As for the dual zone climate control; it's okay, had it in quite a few Golfs now but I reckon I still prefer the older style basic manual heater controls as cabin temperature is easier to regulate and the user is fully in control, not a processor(sp).
