My co-worker has a 2007 6MT GTI. In stock form both the GTI and the 128i put out similar numbers at the track according to Car and Driver. I did think the GTI cornered with a little less body roll but its uglier. But if you need to carry lots of stuff with you remember its a hatchback; said co-worker can carry all his DJ equipment around with him everywhere but there is practically no chance he could do that in my 128i I'm sure.
Now after you chip this car supposedly its a whole different story. He showed me a dyno he did where he's laying down over 200+ HP and 250 lb/ft of torque. Even at the front wheels he claims it let him run the 1/4 at 13.9s at a local track while the best I can do is 14.8s (admittedly I'm not that skilled at the track though). Car and Driver puts the 128i at 14.5s. So if you plan on chipping it the GTI will yield more power for a lot less money than a 128i. Also the power is different...its got more torque at low revs so you can probably get away with less shifting and have alot more useful power around city streets but the 128i is more fun to wind up at the top of the rev range and pull the power out there. Also even with those numbers I think the engine isn't as smooth in the GTI as I'm told there is a small but perceptible turbo lag, nothing like a Subaru I'm sure but another owner of a DSG has told me he feels it.
If money isn't an option I think the 135i in stock form beats the chipped GTI in performance easily and with some small suspension mods equals or betters its handling. Don't forget the brakes on the 135i are heroic.
All and all I'm not disappointed I didn't get a GTI over the 128i one bit but it has dawned on me that it cost a whole lot more.
Hope that helps.