I personally do not like used vehicles for numerous reasons. One being that you never know exactly how well (or poorly) the previous owner treated the car. I would take a new car over an used one...even if it means sacrificing something. However, in this case between the 1.8t and 2.8, u're not really sacrificing anything. In my opinion, the 1.8t is a much better deal....
If it's performance u're after, a chip will push the performance of the 1.8 beyond that of the 2.8. Hope this helps...im a little biased....

I considered exactly the same issue this past summer. And you know where I came out on it! But honestly, I had trouble finding a used A4 with what I wanted on it -- notably a standard transmission and low miles. Then I drove one with the sport package and I had to have that too. Last but not least, I considered the warranty and service plan to be real value, and liked the idea of getting not only a new car with 12 miles on the odo, but also one with the very latest features. I've never doubted that I made the right decision.

Rick
99.5 A4 1.8TQMS

For me it was a matter of options and cashflow more than performance. I'm kind of out of the boy racer phase, so the ability to chip a 1.8T didn't appeal to me. I wanted the smoothness of the 2.8. It had to be manual. It had to have the quattro. I also wanted leather, sunroof and the cold weather package..... Even if I compromised on the 4-cyl, all the other things I wanted were either not available on the 1.8, or priced the 1.8 too far north of my comfort zone.

Truth be told, I was waiting on a Europa Blue 96 2.8QM with everything, under 35K miles and under $22K. So I searched ads and the web for about 9 months until one popped up at Bell Audi this summer for 24K. Compromised on the mileage (36,900) and had to scare up my own CD changer (that was more of a challenge than I thought.... thank you e-bay) but got it for my price. I plan to drive it for at least 5-7 years and right now I couldn't be happier.