Simple advice "Circa £15k" is not going to get you a mk6 170ps gtd. So its 140ps gt mk6 golf or mk5 gt sports in either 140 or 170 form.
I would much rather a mk6 gt 140 than a noisy mk5 140/170 gt sport power delivery is so much better in cr mk6's instead of the peaky delivery of pd's.
Spec wise standard for standard on the mk6 you will also gain cruise control, multifunction steering wheel, mdi(ipod/usb/aux connection), front centre armrest. Although lose automatic headlights, rain sensitive wipers, auto dimming rear view mirror and leather in some later mk5's.