You won't be disappointed in either. I had the GTD for nearly 3 years and my commute was 11 miles and never once had any DPF issues. I think that would only really rear its head if someone was literally just popping to the local shops and never going anywhere else.
The GTD is only really just warming up at that mileage so you don't get the best 60+mpg out of it but I would easily see 50mpg regularly but I could also see 30+mpg depending on how I drove it. On long journeys it would achieve 60+mpg if sensible.
My wife has the GTI and I would regularly take her car to work. Again at 11 miles you won't see the best of its mileage as it excels in longer mileage. It would hit low 30s and sometimes high 20's on the same journey.
In terms of acceleration I had the DTUK box on mine so took it up on par with the BHP of the GTI. So I had the acceleration and the MPG in my favour on the GTD. The power delivery on both is totally different though. Having drove my GTD for a couple of years before the GTI came along, I found when I had the GTI, Nissan Micras and Hyundai i10's were belting away from me at the lights

all because I was driving it like a diesel. If you are coming from a petrol you won't find that though.
Although I loved my GTD and really bonded with the car, there was something I liked about driving the petrol GTI. It just felt more nimble and livelier. A bit like having 2 puppies, one always jumping about full of energy and the other always sleeping

However wake the sleeping one up and away it goes

Interestingly I gave my GTD to my son who was driving a Swift like yourself. The step up in quality is 50 fold. He loves it. Although he was getting 34mpg in his swift I don't think he's getting much more than that in the GTD
