I've had 2 manual GTD's and do 20k per year, mainly A roads and motorway. I use ACC a lot, an awful lot.
Its a piece of cake to drive in manual because the torque is so large.
I avoided DSG because on the Mk7 it was less fuel economical/more CO2/more BIK(!!)
On my current Mk7.5, I avoided DSG because it was more expensive.
Benefits of DSG for me would be:
1) ACC can go right down to 0 mph and back up again - it does work just fine on a manual box, but this would be better - £2k better? Probably not for me even though I don't care about "driving a car" - I love automation.
2) If I drove a lot in heavy traffic an automatic box is definitely better. The few times I slog into cities, I wish I had a DSG but its not that often I do that.
If you are looking at buying a 2nd hand GTD, bear in mind that these things are mainly used by business mile munchers like me (or you probably). From new they are quite reliable IMHO, but buying a 2nd hand one and running it 25k pa.... I think DSG could well become an expensive problem. I wouldn't want to own one getting into the 100k on the clock mark...
I'd also avoid DCC on a 2nd hand one if you are going to mile munch.