The GTI is a stronger, more comfortable, has a classier interior, and just an all around better car than the Si. Don't believe me? Test drive the Si and GTI back to back and tell me what you think. Everything about the car makes it so special: xenon lamps, original GTI styled seats, responsive clutch, intricate buttons and lights, roomy backseat, 60/40 rear folding seats, 6 cd changer with premium sound (product of Bose), free 3 month satellite radio subscription, mpg counter, miles till refuel counter, rear AC, in dash computer menu, and definitely the large amount of torque. I was also surprised when I received my Fast yesterday in the mail.
Torque! The overall driving experience is too much to describe. I also like it when other GTI drivers honk and wave.
Better gas mileage for people with lead feet like myself. Door lock buttons should also light up when unlocked. More powerful rear ac/heater.