Well... The power and traction of the R is something else compared to the GTI espescially in the wet, if you floor it from a standing start in the wet there is absolutely no drama what so ever, it just grips and goes! But thats to be expected really, i've been really thinking about which car i prefer recently and some might think i'm mad (particularly R owners) but i prefer the GTI 
I prefer almost everything about the GTI compared to the R, if your on the motorway there is very little between them when cruising, both very comfortable with DCC, both have plenty of torque in high gears, but imo round town the GTI is better suited, the 'nicer' car to drive? the R seems a little to highly strung for pottering sometimes if that makes sense? I feel i'm using the pudding stirrer a lot just to keep it quiet where having driven a manual mk7 GTI for a day never felt that way. I'm not saying the R is bad by any means it's just more suited to having it's head kicked in which it takes very very well!! 
but i don't drive that way very often so i prefer the GTI's slightly more sedate style.
if your the type of person who likes to potter around in comfort but likes to get on it once in a blue moon but still wants something a bit special then the GTI PP is the one.
Only thing miss about my A3 is the quattro when pulling away in wet much as Jimble describes. Outside of that there is no change in experience in moving to the GTD for rest of drive.
With regards rest of comments I think what you state is pretty much the way most of us drive for a large % of the time. Town and Motorways of which you tend to either move about quite sedately with the traffic or cruise along at speed.
The GTD with DCC and DSG and even the reduced power opposed to a GTI does this superbly and this refinement over the previous surge used to have to manage in the 170BHP Audi is something grown to appreciate not miss.
Not in class of the GTI and then the R for power but you can still push the GTD when there is need and opportunity to and particularly in mid range there is lots to play with - Basically the car fits your 80-90% driving patterns has plenty to offer still when push on.
My next car is in 2016 and already pencilled in a GTI PP because as much as I love the thought of the R power coupled with the 4WD much like Jimble the opportunity to really use it to its capability over what would be sacrificing would not balance out and now having had a GTD for 6 months just having that additional BHP would for me make the GTI the perfect car.
Think something that might stir the pot for me is if a GTD R if ever made.
Given you moved from GTI to R then nobody can accuse you of having an agenda in fact probably the best placed to offer these thoughts.
