Drove a GTD for a short while and was fine on the motorway (and I am fussy re ride quality...). The ride was more noticeable on pot holed city streets. If you are ordering new, you could spec DCC. Also presumably you could get a decent test drive and try all surfaces for yourself?
I'd be more concerned about buying a diesel engine in the current climate - unless you are only keeping it for say 3 years.
The current climate is that now the Arabs have driven American shale fuel companies out of business by artificially reducing the natural price of a barrel of oil, fuel costs are shooting up again - we may only be a few months away from the £1.40/l fuel cost.
The government are trying to persuade people to ditch diesel because it is filthy, when in reality petrol is too, hence particulate filters now being fitted to petrol models and the revelation that all these 1.0 3 cylinder engines that are worked hard all of the time are putting out plenty of NOx themselves and you really aren't saving tge world or your lungs by buying a modern petrol engined car.
The GTD is a fine car, far better when not on Bridgestones (tramping, rock hard riding tyres and a resultant tyre roar).
Diesel is a long way from being dead yet. I'm pleasantly surprised by how much value the wife's A1 1.6TDI has retained as i'm about to chop it in for a Polo GTI+.
All this scaremongering about diesels is putting a lot more tax in government coffers in fuel duty and VAT as we buy more of it.