There very similar really. I had a stainless cat back fitted, weitec 40mm lowering kit all round and some rs4 rep alloys fitted. My brother still has the car as I sold it to him.
My mk5 gti is totally standard just the way i want it.
As for what one I think is better it's hard to say as I liked the feel of the A3 how it drove and the obvious status of it's an "audi". With the golf I love my leather seats altho they burn the skin in the summer sun living here in Devon lol.
I don't like the repair bills tho as just got it's a gti I find everything is a premium haha.
I prefer the look.of the golf tho more than the A3