Being from the US i cant say i've driven either of those engines, but depending on the cost/power ratio of the two, i'd go with the higher output engine. Unless fuel economy is a huge issue, then try the 1.9
There is not a vast difference on the performance of the two cars. I have just ordered a new A6 1.9 TDI(old shape) 20 miles on the clock, multitronic, electric sunroof, leather etc. for a good saving. The 2.5's seem to be alot more expensive and thirstier.
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