Where do you live?
If you live in snow or heavy rain country, the Audi might be a great choice.
Sure it's not as racy as the M3 but if you don't care about that, it's not a consideration.
The Audi has a nicer interior too.
However, as both get bad gas mileage the Audi will be worse.
The M3 will be new however, but getting an 08 you already have the "old" face as the 09 has gotten a facelift.
No doubt a tough choice for someone who's not looking for the top performance of the two.
I'd personally get the M3
But I don't live in snow, nor an area that gets much rain.