Never driven an ST but my mate has one and the GTI is far superior on the inside to the ST.
As for all this "tuning potential" I cant see what the difference is. You can still remap a GTI, change exhaust systems, add larger air intakes etc etc.
Mine has a FTB Miltek and sounds better than my mates ST so thats the sound argument out the window. Dont get me wrong, they are very nice, quick cars but I would choose the GTI all day long till the cows came home.