It very depends what actually you gonna use the car for. 3 litre diesel is a good option. Allroads are usually nicely equipped and they looks stunning in few colours cominations ( with trim all around). Definitely air suspension is more expensive to maintain but comfort itself is worth it. If I was about to buy a6 today I would leave my sLine Quattro avant and would buy allroad
Sline seats are not so comfy, mate. Own one and also have back problems (two bulging discs. One recently removed) and it is not so great sitting too long. Doctors telling me off if driving more than 20 minutes.
Low profile tyres and stiffer suspension don't help either.
Avant is lower to the ground and my wife was nagging me to get something higher to sit down.
She wanted me to sell the A6 and get more family car like Smax. Told her I'll divorce her quicker than sell my beloved A6.
Get the allroad. Air ride will be smoother and it is more to your purpose.
Sline will shake things off you and you will fill it in your back.
Kris is 100% right on this one. I also have disc problems mainly due to being 6'6" and the Le Mans s line doesn't help it at all. Very stiff suspension and very low getting in and out (especially with the seat adjusted all the way down). Unfortunately for my back I love the car but the next one will definitely have to be an allroad!