The mk6 GTD is not in the same class bracket as a GTI. I mean there's Saxo's that beat the GTD. It's around 1.5 secs slower than the GTI and the slouch wont even hit 140. Plus you'll look a cheap ba5tard for not buying a GTI.
One opinion...
I was in convoy with a GTD on the way to Inters this year..nothing in it performance wise at motorway speeds.
My mate also has a Mk5 170 TDI, and there is so little in it on a back road, it's all up to the driver (and the Mk5 170 TDI and Mk6 GTD are so very similar one can be compared to the other performance wise, handling wise etc)
A GTI and GTD are trading gains/losses everywhere...GTI is faster for those who live their lives 14 seconds at a time....or need the extra 15 MPH at jail type speeds...real world, backroads, dual carridgeways, motorways etc, there is little in it.
The GTI is slightly quicker evetywhere if worked...but it is a slight difference, in the real world, where the cars actually drive.