let's say you have a $20k CR-Z... if you sell the A6 privately for $17k then put that down on the CR-Z you will still pay taxes on the $20k price tag and at 7% (not exactly sure what the % is for you) that's an extra $1,400 so that CR-Z becomes $21,400-$17,000=$4,400 you still owe... whereas if you trade in the A6 and get $17k for it you will only pay taxes on the remaining $3k for the CR-Z and @ 7% that's $210+$3,000=$3,210 you owe...
$17k trade-in you owe $3,210
$17k private sale you owe $4,400
these numbers aren't perfect there are a few other things to take into account and of course the $17k is just a base estimate as is the $20k (CR-Z price)...
the question you really have to ask yourself is, "Do i really want a CR-Z?" weigh the advantages of keeping your A6 with the advantages of getting rid of it and getting the CR-Z... see which one makes more sense for you and do what you feel is in your best interest...
personally, i'm a little biased... Audi is entirely too luxury for me and not nearly enough sport... my friend has an '09 A6-S and even that isn't sporty enough for me... so the choice for me would be to simply get rid of the A6 and get the CR-Z...
hope that helps :wavey: