I will only comment on 996 to RS6 transition as everyone above has covered anything else I can think of very well

I came to the RS6 in January 2004, from a 996TTX50.
I can honestly say that in a straight line there is very little difference in performance between the two.
I have the REVO code in my RS6, so 996TTX50 appx. 460-470bhp vs RS6 500-520bhp.
However, in the UK, I find the difference as a "daily vehicle" very marked indeed.
Whilst the 996TT is indeed the ultimate "useable" supercar, it really isn't that practical (as you are now finding

The RS6 is stunningly quick, very composed on all but the worst surfaces, and fairly understated toboot.
I've just returned from a two-week holiday in Cornwall and Devon with dog and the other half's luggage (not an inconsiderable load I might add) and we had an unbelievable amount of space left over.
I couldn't have considered this in the 996...we would've taken the Golf instead

Now whilst I am NOT for a minute saying the RS6 can live with the 996 (TT or other) when it comes to sheer enjoyment, believe me the switch is really is no "wrench" at all, I love mine, so much I have already had a pre-emptive word with several dealers regarding the new one

All in all I personally can't recommend the car highly enough, only problem I have had is with the condensation issue, see the thread relating to it, however if you don't have a Solar Roof then it won't exist in your car.
Go for it is my recommendation
