Recently ordered my A6 4.2 after test driving both. To me, there was no comparison. 12 week delivery from Germany! I have an '00 2.7T and have been very happy with it but have always regretted not getting the 4.2. Since I plan on having it for a while, I don't want to be sorry I didn't, again. If you can afford the extra 10k, you wont be sorry either. However, if you have the patience and can wait 6-12 months, I think the dealers will be more eager to bargain. Right now, there seem to be no deals to be had on the 4.2's.
This is the longest 12 weeks of my life.
Good luck with your decision.