I had a mk7 gtd no mods and now run a mk7 gti pp with a pedal box (a must if you want it to feel fast) I was averaging about 45ish in my gtd and about 32 in the gti...no long runs really, I like the torque of the gtd and the low end power, I preferred the grey seats standard wheels and the grey line through the lights and grill, but I always wanted a gti and I think I always knew that the gtd wouldn't scratch that itch, but they are both good cars.
The gti pp has a lot more grip than the gtd and feels faster from 3000 rpm onwards but not much difference with the pedal box from still to 3000rmp, if it was my money being spent the gti would get the vote but to be honest I really had to think about it and had I had the benefit of running a tuning box on my old gtd it might have got the nod....but that's only because I am still getting use to driving a performance petrol and have to run it at much higher revs than I naturally want to, so to summarise the gtd with a tuning box is only really lacking the handling of the gti pp, but the gti pp would really need a pedal box to get the best out of it.