But there's more to it than just which is 'better' on paper. There's things from value for money, ownership costs, aesthetics, performance etc. to consider. The astra is the best Vauxhall made, ever, simple as. The A3 is a middle of the road Audi.
Of course badge snobbery would put a lot within the A3 without question.
I honestly don't know the answer and wouldn't know without doing decent test drives in both, I certainly wouldn't write off the Astra without giving it a try.