as an 8P owner i'm not sure i'd recommend the A3 over an equivalent Mk5 tbh. i think the interior is a bit better built and generally a nicer place to be (tho this is subjective) but the A3 is not as nice to drive, and earlier ones have smaller rear brakes than the GTI. also me personally i don't really think that quattro is particularly needed, but if you must have it in your life spend a bit more and get an S3.
specifically in regards to the GTI/R32 - as above they are different cars with different plus and minus points. only you will know which one suits you better so test drive them both and make a decision.