What are the differences between the C5 ( A6) and the (A4) aside from the slighty larger size and rear leg room?

 

Well are you comparing just the body styles or would you like to also compare the main two engines you can get: the 2.8 in either or the 1.8T in the A4?

 

Obviously interior trim and options are better in A6. As far as the mechanical things are concerned, A4 had 1.8T and 2.8, and A6 had 2.8, 2.7T, & 4.2 engines. A4s suspension set up is bit more tighter,

 

Having both a 2005 A4 3.0 CVT and a 2000 A6 2.7t, and sometimes driving them both on the same day I can tell you that the A6 is just a lot more car. The seats are more comfortable, quite a bit more leg room in the back (main reason for me to get the A6 because of my tall kids), air conditioning in the back with own controls, I also got a full size spare wheel and not a donut like on the A4. Also 3 adults can fit in the rear seat, not so with the A4.
Again the best way to describe the feeling is that the A6 is a lot more car. I would not buy an A4 again, but an A6 would be very likely the next car in my garage if my 2000 A6 begins to cost too much to keep up.

 

The A6 is very comfortable and a nice blend of touring/sport, but it is a heavy car. Good rear seat room and plenty of trunk space. Lots of nice cabin options. It is very stable and quiet for high speed highway driving. I have 137k miles on it and intend to keep it a long time.

I bought a 2003 A4 3.0 MT6 for my daughter. We looked at several 1.8Ts and other 3.0s before buying it. The 3.0 version comes with a lot nicer cabin and features than the 1.8T. It's a very comfortable car and quite peppy with the 3.0, although lacking the torque surge of the turbo. Frankly, if I did not have to cart a family of 4-5 around fairly regularly, I would be very happy with this vehicle.

Test drive both, in all engine configurations, and make up your own mind.