I swapped my Ed30 for a second hand, 6 month old MK6 GTD just over a year a go.
The pros: fantastic inside, major improvement over the MK5. On long motorway type journeys, it excelled. Smooth for a diesel, good mpg, easily achieveing 50 mpg over the 6 months I had the car.
The cons: it isn't in any shape or form a diesel Gti. Not enough power, but worst of all, the extra weight up front of the diesel engine ruins any thoughts of sporty handling. You just can't for instance attack a roundabout in the GTD in the way you can in the GTI, well you can but ultimately you will end up going straight on if trying to turn right! Mine was a 5 door Carbon Grey. The paint literally fell off it at the mearest hint of gravel/debris touching it. I've never seen a bonnet so badly stone-chipped over such a short period of time. I'm glad I didn't have it over this past winter with all the grit about! Basically the GTD is just not 'exciting' like a Gti.
Mine was a manual, so I can't comment on DSG as never had one, but going back I think it would have been more sensible to buy a GT TDI (140hp) with the 18'' wheels than the GTD. I think the GT is a more 'honest' car, not trying to be something it isn't, whereas the GTD is a bit of a VW marketing ploy, jumping on the GTI bandwagon.
Obviously I know people will disagree with me, particularly if they own a GTD, I think I upset a few people on the Golfgti forum for telling it as it is! But if you wanted to choose between the 2 cars, GTD or GT Tdi, after my experience I would definately save a few quid and buy the latter.