Quote: What would you take 6 from Audi or 6 from Mazda ?
Hard to say. The A6 is very nice. But you're looking at a 3-4 year old car.
With the Mazda6, you could grab a decent 6s brand-new and with your trade-in you'd be at about $15-16K which is just eeking out your budget. If you're willing to buy used you can probably grab a really low mileage fully-loaded for about $18K, minus your trade.
If you don't mind buying something that used, I'd probably get the Audi; but if you want new, or very near new, I'd get the Mazda6.
