Ref the a3 vs golf debate, you're always going to find the bias towards the golf on a golf gti forum.
I do recall when i purchased my gti way back in 2006, there was no s3 option and the audi alternative at the time was the a3 s line tfsi with the same 200ps. Every review i read said the s line chassis was too firm, had poor steering and was better without the 11mm lower suspension. The GTI then arrived a few months later and scooped up almost every car of the year award in every country and every hot hatch comparison award.
I still test drove both, on the same day as they were similar cost and the gti had a 12 month waiting list and the audi was available much sooner. They felt nothing alike, body control, steering feel, ride etc the audi felt like an a to b car and the gti felt sporty and fun.
I recall comparisons with other vag cars always said the gti was better to drive, whether against the cupra, vrs, s3 and so forth.
I've never driven the s3 but reviews back at release always ended with, we'd take the gti, lighter on its feet, much better steering feel and body control. I can't deny i'd fancy a s3 black edition though, I'm sure the traction would make up for any short falls in the chassis department
I'm under no illusions that my 6 year old gti would feel dated in comparison to the newly launched hot hatches, car and technology move forwards very quickly