Have you got the diagrams or part numbers of both?
the new one looks awful in my opinion, what a horrible looking bit of plastic. That's going to look PANTS on the front of my New S3, build week 1.
I have the part numbers for both the Golf and the A3 but I'm not sure they will tell you if they are interchangeable.
Golf - old - Radar Unit - 5Q0 907 541 G
Golf - old - Trim - 5G0 907 461
Golf - new - Radar Unit - 5Q0 907 561 A
Golf - new - Trim - 5G0 907 225 B
A3 - old - Radar Unit - 5Q0 907 541 F
A3 - old - Trim - 8V0 807 224
A3 - new - Radar Unit - 5Q0 907 541 H
A3 - new - Trim - 8V0 907 224A
Quite how significant the letter change at the end of the part is I'm not sure, but it does appear that the units used on the Golf and the A3 and the same or very close apart from the trim.
Make sure you are sitting down when you read this next bit.....
Both old and new units on the Golf cost £1,004.56 for the Radar Unit and £2.90 for the trim
For the A3, again the old and new units are the same price - £1,066.97 for the Radar Unit and £3.05 for the trim.
£62 more when you buy it for an Audi than buying the same thing for a Golf.